Chief supplier of element strong ale naturally retains (6)
I believe the answer is:
galena
'chief supplier of element' is the definition.
(galena is the primary ore of lead)
'strong ale naturally retains' is the wordplay.
'retains' indicates a hidden word.
'GALENA' can be found hidden inside 'strong ale naturally'.
(Other definitions for galena that I've seen before include "Commonest ore of lead - Angle "A" (anag)" , "Lead sulphide mineral" , "Len, GAA produces lead sulphide" , "Angela (anag) - ore producing lead" .)