Greeting that boy will love (5)
I believe the answer is:
hello
'greeting' is the definition.
(I know that hello is a type of greeting)
'that boy will love' is the wordplay.
'that boy' becomes 'he' ().
'will' becomes ''ll' (contracted form, as in 'he'll' - 'he will').
'love' becomes 'o' (love means zero in tennis).
'he'+'ll'+'o'='HELLO'
(Other definitions for hello that I've seen before include "Look at that" , "Expression of greeting" , "Conversation starter" , "Musical with Satchmo and Streisand" , "I say" .)