Free, but not fit for use (5)
I believe the answer is:
avail
'free but not fit for use' is the definition.
'avail' can be an answer for 'use' (I've seen this before). I'm not sure about the 'free but not fit for' bit.
'not fit for use' is the wordplay.
'not' is a deletion indicator.
'fit' becomes 'able' (similar in meaning).
'for use' becomes 'available' (I've seen this before).
'available' with 'able' removed is 'AVAIL'.
Can you help me to learn more?
(Other definitions for avail that I've seen before include "Medal (informal)" , "Make use, take advantage" , "Be of service to" , "Serve a purpose" , "Use to one's advantage" .)