Not caught, relative remains free (7)
I believe the answer is:
unleash
'free' is the definition.
(to unleash is to free or release)
'not caught relative remains' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'caught relative' becomes 'le' (I can't explain this - if you can you should believe this answer much more).
'remains' becomes 'ash' (remains of a fire).
'un'+'le'+'ash'='UNLEASH'
Can you help me to learn more?
(Other definitions for unleash that I've seen before include "Let off the lead" , "Free from control" , "Free from restraint like dog" , "Shun ale (anag.)" , "Set free from restraint, say a dog" .)