Wader is busy in stream, initially (4)
I believe the answer is:
ibis
'wader' is the definition.
(I know that ibis is a type of wader)
'is busy in stream initially' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'is busy in stream' is 'IBIS'.
(Other definitions for ibis that I've seen before include "Revered bird" , "Bird sacred in Ancient Egypt" , "Flier" , "Retreat" , "Winger" .)