Supporter, annoyed at first, has to smile (9)
I believe the answer is:
crossbeam
'smile' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'supporter annoyed at first' is the wordplay.
'supporter' becomes 'beam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'annoyed' becomes 'cross' (cross can mean angry).
'at first' says to put letters next to each other.
'beam' after 'cross' is 'CROSSBEAM'.
'has to' acts as a link.
Can you help me to learn more?
(Another definition for crossbeam that I've seen is " Support".)