Fastens up slit (4)
I believe the answer is:
snip
'slit' is the definition.
(thesaurus)
'fastens up' is the wordplay.
'fastens' becomes 'pins' (pinning is a kind of fastening).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'pins' written backwards gives 'SNIP'.
(Other definitions for snip that I've seen before include "Something cheap or easy to do" , "Cut off a small bit" , "Bargain (informal)" , "vasectomy?" , "Cut with scissors in small strokes" .)