Faithful old lover has to behave (5)
I believe the answer is:
exact
'faithful' is the definition.
(I've seen this before)
'old lover has to behave' is the wordplay.
'old lover' becomes 'ex' (an ex-lover).
'has' says to put letters next to each other.
'to behave' becomes 'act' (acting is a kind of behaving).
'ex'+'act'='EXACT'
(Other definitions for exact that I've seen before include "Finished" , "Not approximate; inflict (revenge) on someone" , "Absolutely right" , "As a right, demand" , "Specific" .)