Fear going topless is a mistake (5)
I believe the answer is:
error
'is 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 first letter removed is 'ERROR'.
(Other definitions for error that I've seen before include "Faulty judgment" , "wrong" , "Howler" , "Slip-up" , "Mistake or fault" .)