Choice over shape ultimately for crest (5)
I believe the answer is:
plume
'for crest' is the definition.
(I've seen this before)
'choice over shape ultimately' is the wordplay.
'choice' becomes 'plum' (both can mean excellent).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'ultimately' indicates one should take the final letters.
The final letter of 'shape' is 'e'.
'plum'+'e'='PLUME'
(Other definitions for plume that I've seen before include "Long soft feather" , "Long cloud of smoke" , "Large showy feather" , "Smoke when rising" , "Bunch of feathers" .)