Heard, in short walk, an aquatic bird (4)
I believe the answer is:
tern
'an aquatic bird' is the definition.
(tern is a kind of aquatic bird)
'heard in short walk' is the wordplay.
'heard in' indicates a 'sounds like' (homophone) clue.
'short walk' becomes 'turn' (I've seen this before).
'turn' sounds like 'TERN'.
(Other definitions for tern that I've seen before include "Seabird related to the gull" , "Pay rent for seabird" , "Rent out this gull" , "flier" , "Seagull" .)