Grim job, getting supporter on board! (9)
I believe the answer is:
sternpost
'supporter on board' is the definition.
(I've seen this before)
'grim job getting' is the wordplay.
'grim' becomes 'stern' (synonyms).
'job' becomes 'post' (post is a kind of job).
'getting' says to put letters next to each other.
'stern'+'post'='STERNPOST'
(Other definitions for sternpost that I've seen before include "Ship's upright to which rudder is attached" , "supporter on board" , "supporter in the main" .)