That man will start to organise welcome (5)
I believe the answer is:
hello
'welcome' is the definition.
(both are greetings)
'that man will start to organise' is the wordplay.
'that man' becomes 'he' (he refers to a man).
'will' becomes 'll' (contracted form, as in 'he'll' - 'he will').
'start to' says to take the initial letters.
The first letter of 'organise' is 'o'.
'he'+'ll'+'o'='HELLO'
(Other definitions for hello that I've seen before include "Introductory greeting" , "Musical with Satchmo and Streisand" , "Welcome" , "opener?" , "Conversation starter" .)