Fixing spike (4)
I believe the answer is:
nail
'fixing spike' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nail that I've seen before include "Amall metal spike" , "Detect or catch (a criminal) (colloq.)" , "Covering on tip of the finger" , "Fasten - detect and expose" , "Growth on digit" .)