Ghost endlessly upset -- oh dear! (4)
I believe the answer is:
oops
'oh dear' is the definition.
(both are said in recognition of a mistake)
'ghost endlessly upset' is the wordplay.
'ghost' becomes 'spook' (synonyms).
'endlessly' means to remove the last letter.
'upset' is a reversal indicator.
'spook' with its final letter taken off is 'spoo'.
'spoo' backwards is 'OOPS'.
(Other definitions for oops that I've seen before include "I've slipped" , "I'm sorry" , "Blunder explanation" , "Exclamation on making minor error" , "Clumsy me!" .)