Matter raised after Mark joins service (5,2)
I believe the answer is:
signs up
'joins service' is the definition.
(as in signing up for the army)
'matter raised after mark' is the wordplay.
'matter' becomes 'pus' (pus is a kind of matter).
'raised' says the letters should be written in reverse (going up in a down clue).
'after' is a charade indicator (letters next to each other).
'mark' becomes 'sign' (synonyms).
'pus' reversed gives 'sup'.
'sup' after 'sign' is 'SIGNS UP'.
(Other definitions for signs up that I've seen before include "volunteers" , "Enrols" , "Recruits" .)