Prominent ornament I put in last (6)
I believe the answer is:
finial
'prominent ornament' is the definition.
(architectural ornament)
'i put in last' is the wordplay.
I cannot really understand how this works, but
'i' could be 'a' and 'a' is found within the answer.
'in' is within the answer.
'last' could be 'f' (I've seen this in other clues) and 'f' is found within the answer.
This explanation may well be incorrect...
(Other definitions for finial that I've seen before include "Ornament on the top of a building" , "Ornament at the top of a spire or column" , "Crowning touch" , "Top decoration" , "Roof ornament" .)