Effective old lover shut up about part (9)
I believe the answer is:
expedient
'effective' is the definition.
(thesaurus)
'old lover shut up about part' is the wordplay.
I cannot quite see how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for expedient that I've seen before include "Fitting" , "Appropriate though not necessarily fair" , "Handy" , "Suited to the circumstances" , "Advantageous in a given situation" .)