Royal family gathering at last (4)
I believe the answer is:
king
'royal' is the definition.
(I've seen this before)
'family gathering at last' is the wordplay.
'family' becomes 'kin' ('kin' can be a synonym of 'family'**).
'at last' suggests the final letters.
The last letter of 'gathering' is 'g'.
'kin'+'g'='KING'
(Other definitions for king that I've seen before include "Honour for man" , "Playing card" , "Male monarch" , "Card, draughts piece" , "eg Wenceslas" .)