Faithful lover abandoned Bill (5)
I believe the answer is:
exact
'faithful' is the definition.
(as in a faithful translation)
'lover abandoned bill' is the wordplay.
I cannot really understand how this works, but
'bill' could be 'act' (I've seen this in other clues) and 'act' is found in the answer.
The remaining letters 'ex' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for exact that I've seen before include "Explicit" , "Spot-on" , "correct in every detail" , "Particular" , "Specific" .)