Left to finish advance (4)
I believe the answer is:
lend
'advance' is the definition.
(I know this)
'left to finish' is the wordplay.
'left' becomes 'l' (common abbreviation).
'to finish' becomes 'end' ('end' can be a synonym of 'finish').
'l'+'end'='LEND'
(Other definitions for lend that I've seen before include "Give the use of for a time" , "Give temporarily" , "Let someone borrow" , "Advance, loan" , "Give (a quality) to" .)