Light show failing to finish: private disappointment (8)
I believe the answer is:
comedown
'disappointment' is the definition.
(comedown can mean a disappointment or anti-climax)
'light show failing to finish private' is the wordplay.
'light show' becomes 'comedy' (light entertainment).
'failing to finish' means to remove the last letter.
'private' becomes 'own' (as in one's own or private belongings).
'comedy' with its final letter taken away is 'comed'.
'comed'+'own'='COMEDOWN'
(Other definitions for comedown that I've seen before include "Decrease" , "Loss of status, disappointment" , "Descent to a lower status" , "lessening of its effect?" , "Decline in status" .)