Bill heading away from agreement (3)
I believe the answer is:
act
'bill' is the definition.
(I've seen this in other clues)
'heading away from agreement' is the wordplay.
'heading away from' means to remove the first letter.
'agreement' becomes 'pact' (a pact is an agreement or deal).
'pact' with its initial letter taken off is 'ACT'.
(Other definitions for act that I've seen before include "Behave like thespian" , "Something done" , "Exploit" , "Do something, in a play?" , "Feat" .)