Biting to get a key free (5)
I believe the answer is:
acrid
'biting' is the definition.
(I've seen this in another clue)
'get a key free' is the wordplay.
'get' says to put letters next to each other.
'key' becomes 'c' (musical key).
'free' becomes 'rid' (to be free of something is to be rid of it).
'a' put next to 'c' is 'ac'.
'ac'+'rid'='ACRID'
'to' acts as a link.
(Other definitions for acrid that I've seen before include "Pungent; caustic" , "Stinging" , "Having a bitter smell or taste" , "Tart" , "Bitter - pungent" .)