Imagine a setup similar to a doubleslit but it has only 1 slit and the other slit is just a reflection on a mirror. Will I still get interference pattern despite only 1 real slit?
Provided that the path length between the two rays does not exceed the coherence length of the light you will see an interference pattern just as if you were using two slits. This is after all just a variant of the usual double slit experiment if we are using an incoherent source and need a single slit before the double slits:
In fact there is a well know experiment that does exactly this called Lloyd's mirror.
(image from Wikipedia)
In principle what better way to produce a coherent sources by having one real source and a mirror image(s) of the real source.
Both sources "emit" waves of the same frequency and provided that the coherence length condition is satisfied where those wave overlap a visible interference pattern will be formed.
For light Lloyd's mirror is a commonly cited example with a real source $S$ and a virtual source $S'$. [Hecht - Optics]
The interference pattern is a result of the superposition of light direct from the source and reflected light from the source.
An adaption of this is Fresnel's inclined mirrors. [Hecht - Optics]
In this case you have the light from two virtual (reflected)sources $S_1$ and $S_2$.
Interference of sound waves occurs when sound is reflected from a series of regularly spaced "mirrors" which may be metal railing, steps, etc.
Hitting a bass drum in a stadium in front of a set of concrete results in the reflected sound having a distinct pitch (frequency) as opposed to the direct sound from the drum having no distinct pitch 9a whole range of frequencies.
In this case you can imagine a whole series of virtual drum images behind the steps spaced at intervals of $2D$ with reinforcement occurring foe particular waves related to the separation of the steps $D = \lambda/2$.
You may have measured the wavelength of sound using standing waves by having sound waves from a loudspeaker $a$ superposing with sound waves reflected from a metal plate $c$ and detecting the nodes and antinodes using a microphone $b$.
Another way of describing the situation is to say that there is a virtual image of the loudspeaker behind the metal reflector and (reflected) waves from that virtual image superpose with the waves direct from the loudspeaker to form an interference pattern.