Staff supporting strike is a killer (6)
I believe the answer is:
hitman
'killer' is the definition.
(I know that hitman is a type of killer)
'staff supporting strike' is the wordplay.
'staff' becomes 'man' (as in staffing or manning a particular place).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'strike' becomes 'hit' (synonyms).
'man' put after 'hit' is 'HITMAN'.
'is a' acts as a link.
(Other definitions for hitman that I've seen before include "killer" , "offer?" , "Hired gun" , "Assassin" .)