Informant about to spin a yarn (6)
I believe the answer is:
canary
'informant' is the definition.
'about to spin a yarn' is the wordplay.
'about to' becomes 'ca' (I've seen this in other clues).
'spin' indicates an anagram.
'yarn' with letters rearranged gives 'nary'.
'ca'+'nary'='CANARY'
(Other definitions for canary that I've seen before include "Bird from an island group" , "Bright yellow shade" , "grass" , "Yellow bird, formerly taken down mines" , "Finch prized for its song" .)