Expression of agreement (6)
'expression of agreement' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for roger that I've seen before include "man, > I understand" , "R in telecommunications" , "- - Moore (007)" , "Chap" , "Military 'OK'" .)
(Other definitions for righto that I've seen before include "Correct" , "very well" , "Informal expression of agreement or acceptance" , "I agree (informal)" , "OK" .)