Reel cut up -- oh dear! (4)
I believe the answer is:
oops
'oh dear' is the definition.
(both are said in recognition of a mistake)
'reel cut up' is the wordplay.
'reel' becomes 'spool' (synonyms).
'cut' means to remove the last letter.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'spool' with its final letter taken away is 'spoo'.
'spoo' back-to-front is 'OOPS'.
(Other definitions for oops that I've seen before include "Apologetic exclamation" , "Oh dear, that's a mistake!" , "- - -a-daisy" , "Exclamation after mistake or minor accident" , "statement after accident" .)