Hawk-headed god exists to save lake birds (5)
I believe the answer is:
rails
'birds' is the definition.
(rail is a kind of bird)
'hawk-headed god exists to save lake' is the wordplay.
'hawk-headed god' becomes 'ra' (Egyptian god).
'exists' becomes 'is' ('be' can be a synonym of 'exist').
'to save' means one lot of letters goes inside another.
'lake' becomes 'l' (geographical abbreviation).
'is' enclosing 'l' is 'ils'.
'ra'+'ils'='RAILS'
(Other definitions for rails that I've seen before include "For trains" , "Makes a protest" , "secretive birds" , "Wading birds - dysfunctional person has come off them" , "Water birds" .)