Birds, say, disturbed rest (6)
I believe the answer is:
egrets
'birds' is the definition.
(egret is a kind of bird)
'say disturbed rest' is the wordplay.
'say' becomes 'eg'.
'disturbed' is an anagram indicator.
'rest' is an anagram of 'rets'.
'eg'+'rets'='EGRETS'
(Other definitions for egrets that I've seen before include "Greets like certain herons" , "Fliers" , "White wading birds" , "Greets like fishing eagles" , "creatures" .)