Starts joint exit illicitly during service (4,4)
I believe the answer is:
jump ship
'exit illicitly during service' is the definition.
(leave a post on a ship)
'starts joint' is the wordplay.
'starts' becomes 'jumps' (to start can mean to jump at a sudden shock).
'joint' becomes 'hip' (part of body).
'jumps'+'hip'='JUMP SHIP'
(Other definitions for jump ship that I've seen before include "Unofficially, leave" , "Desert (originally from navy)" , "(Sailor) desert" , "Defect" .)