You can't keep it forever (6)
I believe the answer is:
rental
'you can't keep it forever' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rental that I've seen before include "money for landlord" , "Charge to hirer" , "Payment to landlord" , "Learnt (anag) -- lease" , "Payment for hire" .)