Lavish old lover to get locked up, we hear (6)
I believe the answer is:
expend
'lavish' is the definition.
(I've seen this before)
'old lover to get locked up we hear' is the wordplay.
'old lover' becomes 'ex' (an ex-lover).
'to get' says to put letters next to each other.
'locked up' becomes 'penned' (penned in or imprisoned).
'we hear' shows a homophone (sound like).
'penned' is a homophone of 'pend'.
'ex'+'pend'='EXPEND'
(Other definitions for expend that I've seen before include "Pay out money" , "Lay out, disburse" , "Consume" , "Spend or use up" , "Use (energy)" .)