Former lover confined welcomes working advocate (8)
I believe the answer is:
exponent
'advocate' is the definition.
(I know that advocate can be written as exponent)
'former lover confined welcomes working' is the wordplay.
'former lover' becomes 'ex' (an ex-lover).
'confined' becomes 'pent' (similar in meaning).
'welcomes' indicates putting letters inside (in sense of 'invites in').
'working' becomes 'on' (on can mean working or activated).
'pent' placed around 'on' is 'ponent'.
'ex'+'ponent'='EXPONENT'
(Other definitions for exponent that I've seen before include "Interpreter - performer" , "index" , "Symbol of power" , "Promoter (of an idea or theory)" , "Propagandist" .)