Rodent initially caught on yellow plant, unfortunately (5)
I believe the answer is:
coypu
'rodent' is the definition.
(coypu is a kind of rodent)
'initially caught on yellow plant unfortunately' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'caught on yellow plant unfortunately' is 'COYPU'.
(Other definitions for coypu that I've seen before include "rodents" , "South American beaver-like beast" , "Creature in fur coat" , "Large South American beaver-like rodent" , "S. American "beaver"" .)