Man will ring, and say this first? (5)
'say this first?' is the definition.
(word said at the beginning of an interaction)
'man will ring' is the wordplay.
'man' becomes 'he' ().
'will' becomes ''ll'.
'ring' becomes 'o'.
'and' is the link.
(Other definitions for hello that I've seen before include "Conventional word of greeting", "Meeting greeting", "glossy magazine", "Look at that", "Conversation starter".)