Groom's passion ultimately consumed (6)
I believe the answer is:
neaten
'groom's' is the definition.
(synonyms)
'passion ultimately consumed' is the wordplay.
'ultimately' indicates one should take the final letters.
'consumed' becomes 'eaten' (similar in meaning).
The last letter of 'passion' is 'n'.
'n'+'eaten'='NEATEN'
(Other definitions for neaten that I've seen before include "Trim" , "Make tidy" , "Groom" , "Straighten up" , "Make orderly" .)