Singer in a stag party goes first (9)
I believe the answer is:
balladeer
'singer' is the definition.
(someone singing ballads)
'a stag party goes first' is the wordplay.
'stag' becomes 'deer' (I've seen this before).
'party' becomes 'ball' (ball is a kind of party).
'goes first' says to put letters next to each other.
'a'+'deer'='adeer'
'adeer' after 'ball' is 'BALLADEER'.
'in' is the link.
(Other definitions for balladeer that I've seen before include "One sings" , "Minstrel" , "One making a song" , "pop singer" .)