Mistake involving hook at end of line (5)
I believe the answer is:
gaffe
'mistake' is the definition.
(gaffe is a kind of mistake)
'hook at end of line' is the wordplay.
'hook' becomes 'gaff' (gaff is a kind of hook).
'at end of' indicates one should take the final letters.
The final letter of 'line' is 'e'.
'gaff'+'e'='GAFFE'
'involving' acts as a link.
(Other definitions for gaffe that I've seen before include "indiscreet remark" , "embarrassing mistake" , "Boob" , "Clanger" , "bad mistake" .)