Militarist marines ahead in punt (9)
I believe the answer is:
warmonger
'militarist' is the definition.
('warmonger' can be a synonym of 'militarist')
'marines ahead in punt' is the wordplay.
'marines' becomes 'rm' (abbreviation for 'Royal Marines').
'ahead' becomes 'on' ('on' can be a synonym of 'ahead').
'in' means one lot of letters goes inside another.
'punt' becomes 'wager' (I've seen this before**).
'rm'+'on'='rmon'
'rmon' going inside 'wager' is 'WARMONGER'.
(Other definitions for warmonger that I've seen before include "Jingoist" , "Jingoistic individual" , "Bellicose type" , "Bellicose person" , "Aggressor" .)