Long journey ends in disappointment for the monk (4)
I believe the answer is:
trek
'long journey' is the definition.
(I've seen this in another clue)
'ends in disappointment for the monk' is the wordplay.
'ends in' says to take the final letters.
The last letters of 'disappointment for the monk' are 'TREK'.
(Other definitions for trek that I've seen before include "Slog" , "Difficult journey on foot" , "Long hard journey on foot" , "Long walk in the mountains?" , "Long difficult walk" .)