Biting Bill to get free (5)
I believe the answer is:
acrid
'biting' is the definition.
(I've seen this before)
'bill to get free' is the wordplay.
'bill' becomes 'ac'.
'to get' says to put letters next to each other.
'free' becomes 'rid' (synonyms).
'ac'+'rid'='ACRID'
(Other definitions for acrid that I've seen before include "Pungent or sharp, like smoke" , "Bitter in smell, like smoke" , "causing corrosion" , "Strong and sharp-smelling" , "Astringent" .)