Big inmate finally escapes and is free (6)
I believe the answer is:
gratis
'free' is the definition.
('gratis' can be a synonym of 'free')
'big inmate finally escapes and is' is the wordplay.
I cannot really see how this works, but
'is' is present in the answer.
The remaining letters 'grat' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
(Other definitions for gratis that I've seen before include "On the house" , "Free of charge" , "Without payment" , "Free, costs nothing" , "Costing nothing, free" .)