Biting when account half paid (4)
I believe the answer is:
acid
'biting' is the definition.
(I've seen this before)
'account half paid' is the wordplay.
'account' becomes 'ac' (abbreviation).
'half' suggests halving the letters.
'paid' cut in half is 'id'.
'ac'+'id'='ACID'
'when' is the link.
(Other definitions for acid that I've seen before include "Corrosive fluid" , "Sour - type of rain or drop" , "Sarcastic - tart" , "something sharp" , "Caustic liquid of many kinds" .)