Bird, the wren, regularly spotted (4)
I believe the answer is:
tern
'bird' is the definition.
(tern is a kind of bird)
'the wren regularly spotted' is the wordplay.
'regularly spotted' indicates alternate letters.
'the'+'wren'='thewren'
The alternate letters of 'thewren' are 'TERN'.
(Other definitions for tern that I've seen before include "flier" , "Small gull with forked tail" , "Rent out the bird" , "Fork-tailed relative of the gull" , "Pay rent for seabird" .)