Drink: boy needs one (7)
I believe the answer is:
martini
'drink' is the definition.
(martini is a kind of drink)
'boy needs one' is the wordplay.
'boy' becomes 'martin' ().
'needs' means one lot of letters go next to another.
'one' becomes 'I' (Roman numeral).
'martin'+'i'='MARTINI'
(Other definitions for martini that I've seen before include "Italian drink" , "In glass, one's dry" , "Type of Italian vermouth" , "Cocktail made with gin or vodka and vermouth" , "Sienese painter" .)