Spy gets kind of crimson once cover's blown (5)
I believe the answer is:
agent
'spy' is the definition.
(secret agent)
'kind of crimson once cover's blown' is the wordplay.
'kind of crimson' becomes 'magenta' (colour).
'once cover's blown' indicates the central letters (the letters covering the rest are removed).
The central letters of 'magenta' are 'AGENT'.
'gets' acts as a link.
(Other definitions for agent that I've seen before include "Instrument" , "One does business on behalf of another" , "One's representative" , "Person acting on behalf of another" , "Spy; service provider" .)