Not entirely rich, getting benefit (4)
I believe the answer is:
plus
'benefit' is the definition.
(I've seen this before)
'not entirely rich' is the wordplay.
'not entirely' means to remove the last letter.
'rich' becomes 'plush' (similar in meaning).
'plush' with its final letter taken off is 'PLUS'.
'getting' acts as a link.
(Other definitions for plus that I've seen before include "mark for arithmetic" , "An advantage (colloq.)" , "Positive thing" , "Additional" , "Distant" .)