Supplement, with "out” (3)
I believe the answer is:
eke
'supplement with out' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eke that I've seen before include "Increase, lengthen" , "Make a living laboriously - a palindrome" , "long ago" , "To -- out is to add to" , "addition to Scots" .)