Starting to serve a boy's dish (5)
I believe the answer is:
salad
'dish' is the definition.
(salad is a kind of dish)
'starting to serve a boy's' is the wordplay.
'starting to' says to take the initial letters.
'boy' becomes 'lad' (informal term for boy).
The first letter of 'serve' is 's'.
's'+'a'+'lad'='SALAD'
(Other definitions for salad that I've seen before include "Healthy dish" , "one gets dressed" , "One may well be green" , "Raw, green, veg" , "Cold dish of various vegetables," .)