Fellow will ring for a "how do you do?" (5)
I believe the answer is:
hello
'a how do you do?' is the definition.
(synonyms)
'fellow will ring' is the wordplay.
'fellow will' becomes 'he'll' (he will).
'ring' becomes 'O' (letter resembling a ring).
'hell'+'o'='HELLO'
'for' is the link.
(Other definitions for hello that I've seen before include "Musical with Satchmo and Streisand" , "Meeting greeting" , "Look at that" , "Surprised cry" , "Conversation starter" .)