Cut spikes up (4)
I believe the answer is:
'cut' is the definition.
(I know that snip is a type of cut)
'spikes up' is the wordplay.
'spikes' becomes 'pins' (pinning is a kind of spiking).
'up' is a reversal indicator (in down clue: letters go upwards).
'pins' written backwards gives 'SNIP'.
(Other definitions for snip that I've seen before include "Bargain (informal)" , "Cut with scissors in small strokes" , "Quick cut - something easily done" , "Surprisingly cheap item" , "Cut with scissors - bargain" .)