White bird, say tailless tern ascending (5)
I believe the answer is:
egret
'white bird' is the definition.
(I've seen this before)
'say tailless tern ascending' is the wordplay.
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'tailless' means to remove the last letter (the tail of the word is removed).
'ascending' is a reversal indicator.
'tern' with its last letter taken away is 'ter'.
'ter' backwards is 'ret'.
'eg'+'ret'='EGRET'
(Other definitions for egret that I've seen before include "young bird" , "Kind of white heron - greet it" , "Type of heron" , "One of a pair of waders" , "Long-plumed heron" .)