Small plant overproduces some parts (4)

I believe the answer is:

moss

I'm a little stuck... Click here to teach me more about this clue!

'small plant' is the definition.
'moss' can be an answer for 'plant' (I've seen this before). I am not sure about the 'small' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for moss that I've seen before include "evidence of neglect?" , "that may hug trees?" , "Plant(s) not wanted on lawns" , "Plant growing in dense mats" , "Plant reproducing by spores" .)

I've seen this clue in the Sydney Morning Herald.

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