Bird in water gently rising (5)
I believe the answer is:
egret
'bird' is the definition.
(egret is a kind of bird)
'in water gently rising' is the wordplay.
'in' indicates the answer is hidden within the clue.
'rising' is a reversal indicator (going up in a down clue).
'terge' can be found hidden inside 'water gently'.
'terge' written backwards gives 'EGRET'.
(Other definitions for egret that I've seen before include "flier" , "Mainly white heron" , "Water-bird, has plume" , "Greet (anag) -- wading bird" , "Wading bird like a heron" .)