Stuck in the mud, it can't fly (3)
I believe the answer is:
emu
'it can't fly' is the definition.
(emus are flightless birds)
'stuck in the mud' is the wordplay.
'stuck in' indicates the answer is hidden within the clue.
'EMU' is hidden in the letters of 'the mud'.
(Other definitions for emu that I've seen before include "Fast runner in Oz" , "Native Australian bird" , "Australian bird related to the ostrich" , "unit of measurement" , "it'll never fly!" .)