Refine garden Richelieu used to an extent (6)
I believe the answer is:
enrich
'refine' is the definition.
(thesaurus)
'garden richelieu used to an extent' is the wordplay.
'to an extent' indicates the answer is hidden within the clue.
'ENRICH' can be found hidden inside 'garden richelieu used'.
(Other definitions for enrich that I've seen before include "Give money" , "increase wealth" , "Concentrate" , "Adorn, enhance or make wealthy" , "develop" .)