Heron, say, on top of tree (5)
I believe the answer is:
egret
'heron' is the definition.
(egret is a kind of heron)
'say on top of tree' is the wordplay.
'say' becomes 'eg'.
'on' becomes 're' ('on' can be mean 'with reference to').
'top of' suggests taking the first letters.
The first letter of 'tree' is 't'.
'eg'+'re'+'t'='EGRET'
(Other definitions for egret that I've seen before include "Mainly white heron" , "Wading bird like a heron" , "Bird eating fish" , "Lesser white heron" , "Greet (anag) -- wading bird" .)