If coming back in poor visibility, you may find a square peg (6)
I believe the answer is:
misfit
'a square peg' is the definition.
'if coming back in poor visibility you' is the wordplay.
'coming back' says the letters should be written in reverse.
'in' is an insertion indicator.
'poor visibility you' becomes 'mist' (I can't explain this - if you can you should believe this answer much more).
'if' in reverse letter order is 'fi'.
'fi' inserted into 'mist' is 'MISFIT'.
'may find' is the link.
Can you help me to learn more?
(Other definitions for misfit that I've seen before include "One can't adapt to their circumstances" , "Someone not suited" , "An unsuitable thing" , "Weirdo" , "Maverick, nonconformist" .)