Dash back for fix (3)
I believe the answer is:
pin
'for fix' is the definition.
(to pin is to fix in place)
'dash back' is the wordplay.
'dash' becomes 'nip' (both can mean a small amount).
'back' says the letters should be written backwards.
'nip' reversed gives 'PIN'.
(Other definitions for pin that I've seen before include "Badge" , "Initially, confidential number for use of bank card" , "Small object to fasten or attach" , "Debit-card number" , "Stick with flag placed in golf hole" .)