Grouse fly east, retaining height (6)
I believe the answer is:
whinge
'grouse' is the definition.
(I've seen this before)
'fly east retaining height' is the wordplay.
'fly' becomes 'wing' ('wing' can be a synonym of 'fly'**).
'east' becomes 'e' ('E' can be a synonym of 'east').
'retaining' is an insertion indicator.
'height' becomes 'h'.
'wing'+'e'='winge'
'winge' enclosing 'h' is 'WHINGE'.
(Other definitions for whinge that I've seen before include "Grumble or complain persistently" , "Hewing (anag.)" , "Peevish complaint" , "Complain annoyingly" , "Moan" .)