Disappointment as I discovered wild Manx cat roaming (10)
I believe the answer is:
anticlimax
'disappointment' is the definition.
(I've seen this before)
'i discovered wild manx cat roaming' is the wordplay.
'discovered' means to look at the middle letters (dis-covered - outer letters removed).
'roaming' indicates anagramming the letters.
The central letters of 'wild' are 'il'.
'i'+'il'+'manx'+'cat'='iilmanxcat'
'iilmanxcat' anagrammed gives 'ANTICLIMAX'.
'as' acts as a link.
(Other definitions for anticlimax that I've seen before include "Disappointing end" , "disappointment" , "Let-down after excitement" , "Come-down" , "Disappointing ending" .)