Compares as some say compound plants (7)
I believe the answer is:
'compares' is the definition.
Although both the answer and definition are plural nouns, I don't see how they can define each other.
'some say compound plants' is the wordplay.
'some' says to take the centre.
'say' becomes 'click' (clicking is a kind of saying).
'compound plants' becomes 'hens' (I can't justify this - if you can you should give a lot more credence to this answer).
The middle of 'click' is 'lic'.
'as' acts as a link.
Can you help me to learn more?
(Another definition for lichens that I've seen is " Tiny grey and green plant growths on rocks and tree-trunks".)