Dolly Parton's behind first benefit (6)
I believe the answer is:
assist
'benefit' is the definition.
(I've seen this before)
'dolly parton's behind first' is the wordplay.
I cannot quite see how this works, but
'behind' could be 'ass' (both can mean one's back side) and 'ass' is found in the answer.
'first' could be 'ist' (ist resembles 1st) and 'ist' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for assist that I've seen before include "Work as a subordinate" , "Work with N help" , "Expedite" , "Help to make" , "provide backing" .)