One calls round after six to relax, right? (7)
I believe the answer is:
visitor
'one calls' is the definition.
'round after six to relax right?' is the wordplay.
'round' becomes 'o' (round can mean a round object**).
'after' says to put letters next to each other.
'six' becomes 'vi' ('vi' can be a synonym of 'six').
'to relax' becomes 'sit' (sitting is a type of relaxing).
'right?' becomes 'r' (common abbreviation).
'vi'+'sit'='visit'
'o' after 'visit' is 'visito'.
'visito'+'r'='VISITOR'
(Other definitions for visitor that I've seen before include "Person turning up" , "who might stay?" , "One coming calling" , "Caller, guest" , "Company" .)