Assigned clerk to fill in advert (8)
I believe the answer is:
ascribed
'assigned' is the definition.
('ascribe' can be a synonym of 'assign')
'clerk to fill in advert' is the wordplay.
'clerk' becomes 'scribe' (I have seen 'Medieval clerk' mean 'scribe' so perhaps 'clerk' could also mean 'scribe').
'to fill in' indicates putting letters inside.
'advert' becomes 'ad' (synonyms).
'scribe' placed into 'ad' is 'ASCRIBED'.
(Other definitions for ascribed that I've seen before include "Put down" , "Attributed, regarded as originated by" , "Given the attribution" .)