Bright and pleasant cry of approval unknown (6)
I believe the answer is:
cheery
'bright and pleasant' is the definition.
(I've seen this before)
'cry of approval unknown' is the wordplay.
'cry of approval' becomes 'cheer' (I've seen this before).
'unknown' becomes 'y' (algebra).
'cheer'+'y'='CHEERY'
(Other definitions for cheery that I've seen before include "Joyful -- optimistic" , "Happy, optimistic" , "In good spirits" , "Bright - pleasant - good-humoured" , "Merry, jovial" .)