He will get old greeting (5)
I believe the answer is:
hello
'greeting' is the definition.
(hello is a kind of greeting)
'he will get old' is the wordplay.
'will' becomes 'll' (contracted form, as in 'he'll' - 'he will').
'get' says to put letters next to each other.
'old' becomes 'o' (abbreviation).
'he'+'ll'+'o'='HELLO'
(Other definitions for hello that I've seen before include "Conventional word of greeting" , "royals dominate its pages?" , "Expression of greeting" , "glossy magazine" , "Hi, howdy" .)