Tailless aquatic bird found in grass, weakened (7)
I believe the answer is:
reduced
'weakened' is the definition.
(reducing is a kind of weakening)
'tailless aquatic bird found in grass' is the wordplay.
'tailless' means to remove the last letter.
'aquatic bird' becomes 'duck' (duck is a kind of aquatic bird).
'found in' means one lot of letters goes inside another.
'grass' becomes 'reed' (I've seen this before**).
'duck' with its final letter taken away is 'duc'.
'duc' going into 'reed' is 'REDUCED'.
(Other definitions for reduced that I've seen before include "Curtailed" , "Got rid of some bodily weight" , "Abridged" , "Discounted" , "Impoverished" .)