Bird frequenting quagmire, mudlark (3)
I believe the answer is:
emu
'bird' is the definition.
(emu is a kind of bird)
'frequenting quagmire mudlark' is the wordplay.
'frequenting' indicates the answer is hidden within the clue (to frequent can mean to visit something).
'EMU' is hidden in the letters of 'quagmire mudlark'.
(Other definitions for emu that I've seen before include "Large Australian flightless bird" , "Ratite" , "Bird of tremulous kind" , "Extinct bird" , "Australian running bird." .)