Blabbing finally, informer is free (6)
I believe the answer is:
gratis
'free' is the definition.
(synonyms)
'blabbing finally informer is' is the wordplay.
'finally' says to take the final letters.
'informer' becomes 'rat' (synonyms).
The final letter of 'blabbing' is 'g'.
'g'+'rat'+'is'='GRATIS'
(Other definitions for gratis that I've seen before include "Free of charge" , "Free, costs nothing" , "not charged" , "Free (Latin)" , "Free, at no cost" .)