Increase, lengthen (3)
I believe the answer is:
eke
I believe this is a double definition.
'increase' is the first definition.
(I've seen this before)
'lengthen' is the second definition.
(I've seen this in another clue)
(Other definitions for eke that I've seen before include "Make last longer" , "To -- out is to add to" , "Make a living laboriously - a palindrome" , "Stretch (out); in addition (archaic)" , "Increase, lengthen" .)