Say hello: regent to shake with GI (8)
I believe the answer is:
greeting
'say hello' is the definition.
(I know that hello is a type of greeting)
'regent to shake with gi' is the wordplay.
'to shake with' indicates anagramming the letters.
'regent' anagrammed with 'gi' makes 'GREETING'.
(Other definitions for greeting that I've seen before include "Welcoming" , "Word of welcome" , "Word(s) of welcome" , "Words of salutation" , "Hello" .)