Untold pounds pocketed by earl's wife (9)
I believe the answer is:
countless
'untold' is the definition.
(I know that untold can be written as countless)
'pounds pocketed by earl's wife' is the wordplay.
'pounds' becomes 'l' (abbreviation e.g. Lsd - pounds, shillings, pence).
'pocketed by' indicates putting letters inside.
'earl's wife' becomes 'countess' (I've seen this before).
'l' going inside 'countess' is 'COUNTLESS'.
(Other definitions for countless that I've seen before include "Untold" , "Legion seeming to be" , "not so important" , "Infinite" , "enormous number" .)