Purser’s upset about one unexpected revelation (8)
I believe the answer is:
surprise
'unexpected revelation' is the definition.
(a surprise is a revelation)
'purser's upset about one' is the wordplay.
'upset' is an anagram indicator.
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'pursers' is an anagram of 'surprse'.
'surprse' enclosing 'i' is 'SURPRISE'.
(Other definitions for surprise that I've seen before include "Unexpected thing" , "Shock" , "is not what's expected" , "Startle, astonish" , "Take unawares" .)