It will flatten a canary (6)
I believe the answer is:
roller
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'will flatten a canary' is the wordplay.
'will' says to put letters next to each other (I've seen this in other clues).
'flatten' becomes 'roll' (rolling is a kind of flattening).
'a canary' becomes 'er' (I can't explain this - if you can you should give a lot more credence to this answer).
'roll' put next to 'er' is 'ROLLER'.
Can you help me to learn more?
(Other definitions for roller that I've seen before include "Large sea wave and hair-curler" , "Heavy sea wave and hair curler" , "Long heavy sea-wave" , "flattens lawn" , "Long heavy sea wave" .)