Darling Bill will make a bloomer (5,7)
I believe the answer is:
sweet william
'a bloomer' is the definition.
(sweet William is a kind of flowering plant)
'darling bill' is the wordplay.
'darling' becomes 'sweet' (I've seen this in another clue).
'bill' becomes 'william' (Bill is short for William).
'sweet'+'william'='SWEET-WILLIAM'
'will make' acts as a link.
(Other definitions for sweet william that I've seen before include "Plant with clusters of coloured flowers - WWI allies met (anag)" , "Garden pink -- Willie wet Sam (anag)" , "Garden pink with clusters of vivid flowers" , "Eurasian plant with fragrant flowers" , "Flowering plant - Prince at sixteen?" .)