Sweet but strange infant, shortly adult (3,4)
I believe the answer is:
rum baba
'sweet' is the definition.
(I know that rum baba is a type of sweet)
'strange infant shortly adult' is the wordplay.
'strange' becomes 'rum' (synonyms).
'infant' becomes 'babe' (synonyms).
'shortly' means to remove the last letter.
'adult' becomes 'a' (abbreviation).
'babe' with its final letter taken off is 'bab'.
'rum'+'bab'+'a'='RUM BABA'
'but' is the link.
(Other definitions for rum baba that I've seen before include "Rich cake" , "Yeasty cake soaked in spirit" , "Sweet" , "sort of tipsy-cake" , "Cake soaked in syrup" .)