Monarch's family heading to gala (4)
I believe the answer is:
king
'monarch's' is the definition.
(male monarch)
'family heading to gala' is the wordplay.
'family' becomes 'kin' (synonyms).
'heading to' indicates taking the first letters (the head of the word).
The first letter of 'gala' is 'g'.
'kin'+'g'='KING'
(Other definitions for king that I've seen before include "Play set in medieval Britain" , "Supreme ruler" , "One on throne" , "Eponymous giant ape in movie" , "Court card" .)