Bird bath oddly filled by one bird (7)
I believe the answer is:
bittern
'bird' is the definition.
(bittern is a kind of bird)
'bath oddly filled by one bird' is the wordplay.
'oddly' indicates alternate letters.
'filled by' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'bird' becomes 'tern' (tern is a kind of bird**).
The alternating letters of 'bath' are 'bt'.
'bt' enclosing 'i' is 'bit'.
'bit'+'tern'='BITTERN'
(Other definitions for bittern that I've seen before include "Bird of marshes" , "Marshland bird" , "Aquatic bird" , "Bird that booms" , "inhabitant of marsh" .)