Something sweet, that is, for young Jim (5)
I believe the answer is:
jamie
'young jim' is the definition.
The definition suggests a singular noun which matches the answer.
'something sweet that is' is the wordplay.
I cannot really understand how this works, but
'sweet' could be 'jam' (jam is a kind of sweet) and 'jam' is found in the answer.
'that is' could be 'ie' (short for Latin 'id est', 'that is') and 'ie' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Another definition for jamie that I've seen is " -- Oliver, chef".)