Good artist finally securing very large advance (5)
I believe the answer is:
posit
'advance' is the definition.
(both can mean to put forward or suggest)
'good artist finally securing very large' is the wordplay.
'good' becomes 'pi' (old-fashioned term for pious).
'finally' suggests the final letters.
'securing' means one lot of letters goes inside another.
'very large' becomes 'os' (abbreviation for oversize).
The final letter of 'artist' is 't'.
'pi'+'t'='pit'
'pit' enclosing 'os' is 'POSIT'.
(Other definitions for posit that I've seen before include "advance" , "Put forward (argument)" , "Lay down or assume (as the basis for an argument)" , "Suggest" , "Dispose" .)