Heron, say, on far side of moat (5)
I believe the answer is:
egret
'heron' is the definition.
(egret is a kind of heron)
'say on far side of moat' is the wordplay.
'say' becomes 'eg'.
'on' becomes 're' ('on' can be mean 'with reference to').
'far side of' suggests the final letters.
The last letter of 'moat' is 't'.
'eg'+'re'+'t'='EGRET'
(Other definitions for egret that I've seen before include "Seabird" , "White wading bird" , "young bird" , "White, plumed waterbird" , "Greet (anag) -- wading bird" .)