Key informant is free (6)
I believe the answer is:
gratis
'free' is the definition.
('gratis' can be a synonym of 'free')
'key informant is' is the wordplay.
'key' becomes 'g' (I've seen this in other clues).
'informant' becomes 'rat' (rat is a kind of informant).
'g'+'rat'+'is'='GRATIS'
(Other definitions for gratis that I've seen before include "not charged" , "Without charge" , "Free of charge" , "Without payment" , "Costing nothing, free" .)