Darwin's regularly trimmed beard (3)
I believe the answer is:
awn
'beard' is the definition.
(both can mean a growth on plants)
'darwin's regularly trimmed' is the wordplay.
'regularly trimmed' means one should take alternating letters.
The alternate letters of 'darwins' are 'AWN'.
(Other definitions for awn that I've seen before include "Bristle growing on barley" , "Beard growing from an ear of barley or rye" , "Bristles" , "Stiff bristle on grasses" , "Bristle on grass spikelet" .)