Allow to borrow (4)
I believe the answer is:
lend
'allow to borrow' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lend that I've seen before include "Give on condition of return" , "Supply (money) at interest" , "Give (a quality) to" , "Give the use of for a time" , "contribute" .)