My mistake cutting end off spool when returning (4)
I believe the answer is:
oops
'my mistake' is the definition.
(oops may be said to acknowledge a mistake)
'cutting end off spool when returning' is the wordplay.
'cutting end off' means to remove the last letter.
'when returning' shows that the letters should be reversed in order.
'spool' with its final letter taken away is 'spoo'.
'spoo' reversed gives 'OOPS'.
(Other definitions for oops that I've seen before include "Exclamation of surprise or apology when making mistake" , "statement after accident" , "I've made a mistake!" , "My bad" , "Exclamation of making a blunder" .)