Allocate English vessel to take on board a Royal Marine (7)
I believe the answer is:
earmark
'allocate' is the definition.
(thesaurus)
'english vessel to take on board a royal marine' is the wordplay.
'english' becomes 'E' (abbreviation).
'vessel' becomes 'ark' (ark is a kind of vessel).
'to take on board' means one lot of letters goes inside another.
'royal marine' becomes 'RM' (abbreviation).
'e'+'ark'='eark'
'a'+'rm'='arm'
'eark' going around 'arm' is 'EARMARK'.
(Other definitions for earmark that I've seen before include "Set aside for later" , "Reserve specially" , "Set aside specifically" , "It identifies the outcome" , "'Reserve, set aside (7)'" .)