Fear going topless may be a mistake (5)
I believe the answer is:
error
'be a mistake' is the definition.
(I've seen this before)
'fear going topless' is the wordplay.
'fear' becomes 'terror' (terror is a kind of fear).
'going topless' means to remove the first letter.
'terror' with its initial letter taken off is 'ERROR'.
'may' acts as a link.
(Other definitions for error that I've seen before include "Fallacy, mistake" , "Faux pas" , "Boob" , "heresy" , "Blunder or mistake" .)