Mock royal receiving grand old man (6)
I believe the answer is:
codger
'old man' is the definition.
(codger is a kind of old man)
'mock royal receiving grand' is the wordplay.
'mock' becomes 'cod' (cod can mean false or inauthentic).
'royal' becomes 'ER' (abbreviation for Elizabeth Regina).
'receiving' indicates putting letters inside.
'grand' becomes 'g' (G is short for grand as in a thousand).
'cod'+'er'='coder'
'coder' enclosing 'g' is 'CODGER'.
(Other definitions for codger that I've seen before include "A senior" , "(Old) man" , "Old person" , "Old-fashioned geezer" , "fellow" .)