Man on board to secure big gun (7)
I believe the answer is:
kingpin
'big gun' is the definition.
(I know that kingpin is a type of big gun)
'man on board to secure' is the wordplay.
'man on board' becomes 'king' (chess).
'to secure' becomes 'pin' (pinning is a kind of securing).
'king'+'pin'='KINGPIN'
(Other definitions for kingpin that I've seen before include "Main bolt" , "Supremo" , "25' [TOP BANANA]" , "Crucial person in organisation" , "Essential person or thing" .)