Piece on spike for big gun (7)
I believe the answer is:
kingpin
'big gun' is the definition.
(I know that kingpin is a type of big gun)
'piece on spike' is the wordplay.
'piece' becomes 'king' (king is a kind of chess piece).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'spike' becomes 'pin' (pinning is a kind of spiking).
'king'+'pin'='KINGPIN'
'for' is the link.
(Other definitions for kingpin that I've seen before include "Supremo" , "Most important member of organisation" , "Essential person or thing" , "Pink gin for indispensable person" , "Crucial person in organisation" .)