A greeting the chap is going to love (5)
I believe the answer is:
hello
'a greeting' is the definition.
(hello is a kind of greeting)
'the chap is going to love' is the wordplay.
'the chap' becomes 'he' ().
'is going to' becomes 'll' (as in he'll, we'll etc).
'love' becomes 'o' (love means zero in tennis).
'he'+'ll'+'o'='HELLO'
(Other definitions for hello that I've seen before include "Publication" , "I say" , "Look at that" , "Conventional word of greeting" , "Conversation starter" .)