Big wheel in London to call meeting of viewers (3,7)
I believe the answer is:
eye contact
'meeting of viewers' is the definition.
'big wheel in london to call' is the wordplay.
'big wheel in london' becomes 'eye' (London Eye).
'to call' becomes 'contact'.
'eye'+'contact'='EYE CONTACT'
(Other definitions for eye contact that I've seen before include "direct observation?" , "shared vision" , "Exchange of views" , "Communication" .)