Title-holder wants hot bloomer (7)
I believe the answer is:
campion
'bloomer' is the definition.
(I know that campion is a type of bloomer)
'title-holder wants hot' is the wordplay.
'title-holder' becomes 'champion' (synonyms).
'wants' indicates named letters should be taken away ('want' can mean 'lack').
'hot' becomes 'h' (abbreviation, eg on taps).
'champion' with 'h' taken away is 'CAMPION'.
(Other definitions for campion that I've seen before include "Small flower; Thomas -, Elizabethan poet" , "Film director" , "Wild flower (red, white or bladder)" , "Here's one blooming" , "Edmund ---, martyr; Thomas ---, poet/lute composer" .)