Paper determined to reel in ambassador (5)
I believe the answer is:
sheet
'paper' is the definition.
(sheet is a kind of paper)
'determined to reel in ambassador' is the wordplay.
'determined' becomes 'set' ('set' can be a synonym of 'determined').
'to reel in' means one lot of letters goes inside another.
'ambassador' becomes 'he' (His or Her Excellency).
'set' enclosing 'he' is 'SHEET'.
(Other definitions for sheet that I've seen before include "Bed cover" , "Set of unseparated stamps" , "One of pair tucked in" , "Piece of paper or bedlinen" , "Piece of paper or bedding" .)