This is dropped by plants for birds (8)
I believe the answer is:
thrushes
'birds' is the definition.
(thrush is a kind of bird)
'this is dropped by plants' is the wordplay.
'dropped' indicates named letters should be taken away.
'by plants' becomes 'rushes' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'this' with 'is' taken away is 'th'.
'th'+'rushes'='THRUSHES'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for thrushes that I've seen before include "Songbirds with brownish backs and speckled breasts" , "She hurts the songbirds" , "Birds" .)