Singing birds are able to sign (8)
I believe the answer is:
canaries
'singing birds' is the definition.
'canaries' can be an answer for 'birds' (canary is a kind of bird). I'm not certain of the 'singing' bit.
'able to sign' is the wordplay.
'able' becomes 'can'.
'to sign' becomes 'Aries' (star sign).
'can'+'aries'='CANARIES'
'are' is the link.
Can you help me to learn more?
(Other definitions for canaries that I've seen before include "Islands and yellowish birds" , "The birds forewarn of danger in the mines" , "The birds can arise" , "Mediterranean islands and yellow birds" , "Atlantic islands - cagebirds" .)