A bird stuck in the mud (3)
I believe the answer is:
emu
'a bird' is the definition.
(emu is a type of bird)
'stuck in the mud' is the wordplay.
'stuck in' says the answer is hidden in the clue.
'EMU' is hidden in the letters of 'the mud'.
(Other definitions for emu that I've seen before include "Long-necked bird" , "Ratite" , "it can't get off the ground" , "Aussie bird" , "Big bird" .)