It's possibly blooming nice prince? (5,7)
I believe the answer is:
'it's possibly blooming' is the definition.
(sweet william is a type of flower)
'nice prince?' is the wordplay.
'nice' becomes 'sweet' (both can mean agreeable or pleasant).
'prince?' becomes 'William' (Prince William).
(Other definitions for sweet william that I've seen before include "bloomer" , "Eurasian plant with fragrant flowers" , "Flower (nice boy!)" , "Garden pink -- Willie wet Sam (anag)" , "Flowering plant - Prince at sixteen?" .)