A cyclist's greeting, in the main (4)
I believe the answer is:
ahoy
'greeting in the main' is the definition.
(greeting at sea - the main can mean the sea)
'a cyclist's' is the wordplay.
'cyclist' becomes 'hoy' (Sir Chris Hoy).
'a'+'hoy'='AHOY'
(Other definitions for ahoy that I've seen before include "Nautical call for attention" , "The sailor cries ''ship . . . .''" , "What a sailor may" , "Hello there on board!" , "Coastal land" .)