Classes rise to welcome king and son (6)
I believe the answer is:
groups
'classes' is the definition.
(grouping is a kind of classing)
'rise to welcome king and son' is the wordplay.
'rise' becomes 'goup' (going up is a kind of rising**).
'to welcome' is an insertion indicator (in sense of 'inviting in').
'king' becomes 'r' (r is abbreviation for rex).
'and' is a charade indicator (letters next to each other).
'son' becomes 's'.
'goup' placed around 'r' is 'group'.
'group'+'s'='GROUPS'
(Other definitions for groups that I've seen before include "Numbers of entities considered as units" , "Numbers gathered or classed together" , "Numbers of associated individuals" , "Lots of" , "Numbers of people or things classed together" .)