Page ambassador to join group (5)
I believe the answer is:
sheet
'page' is the definition.
(I've seen this in another clue)
'ambassador to join group' is the wordplay.
'ambassador' becomes 'HE' (His or Her Excellency).
'to join' means one lot of letters goes inside another (inserted letters join the word).
'group' becomes 'set' (both can mean a collection).
'he' inserted into 'set' is 'SHEET'.
(Other definitions for sheet that I've seen before include "Piece of paper or bedding" , "Set of unseparated stamps" , "Bed cover" , "Piece of paper or bedlinen" , "newspaper" .)