Not an Irish county crop (4)
I believe the answer is:
trim
'crop' is the definition.
(synonyms)
'not an irish county' is the wordplay.
'not' indicates named letters should be taken away.
'irish county' becomes 'antrim' (County Antrim in Ireland).
'antrim' with 'an' taken out is 'TRIM'.
(Other definitions for trim that I've seen before include "in good condition" , "border" , "(Make) neat and tidy" , "Neat - crop" , "Spick and span" .)