Square peg in a round hole (6)
I believe the answer is:
misfit
'square peg in a round hole' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for misfit that I've seen before include "An oddity, doesn't integrate with others" , "Weirdo" , "awkward one?" , "Fish out of water" , "not a suitable one" .)