Trim nails back (4)
I believe the answer is:
snip
'trim' is the definition.
(I know that trim can be written as snip)
'nails back' is the wordplay.
'nails' becomes 'pins'.
'back' says the letters should be written in reverse.
'pins' back-to-front is 'SNIP'.
(Other definitions for snip that I've seen before include "Cut; bargain (colloq.)" , "Cut with scissors in small strokes" , "Cut quickly with a scissors" , "bargain buy" , "Cut using pins" .)