Cheer a blunder in attempt at double-dealing (9)
I believe the answer is:
treachery
'double-dealing' is the definition.
(I've seen this before)
'cheer a blunder in attempt' is the wordplay.
'blunder' is an anagram indicator.
'in' indicates putting letters inside.
'attempt' becomes 'try' (synonyms).
'cheer'+'a'='cheera'
'cheera' anagrammed gives 'reache'.
'reache' put into 'try' is 'TREACHERY'.
'at' is the link.
(Other definitions for treachery that I've seen before include "Two-timing" , "Betrayal of trust" , "Perfidious behaviour" , "lack of loyalty" , "perfidious conduct" .)