Militaryman posted guarding explosive gear (8)
I believe the answer is:
sergeant
'militaryman' is the definition.
'posted guarding explosive gear' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post'**).
'guarding' is an insertion indicator.
'explosive' indicates an anagram.
'gear' anagrammed gives 'rgea'.
'sent' placed around 'rgea' is 'SERGEANT'.
(Other definitions for sergeant that I've seen before include "Reagents for army NCO" , "NCO; Death a fell one, Hamlet" , "Rank above corporal" , "Soldier and lover in Hardy novel" , "Army rank" .)