Bird on wall at regular intervals (3)
I believe the answer is:
owl
'bird' is the definition.
(owl is a kind of bird)
'on wall at regular intervals' is the wordplay.
'at regular intervals' indicates alternate letters.
'on'+'wall'='onwall'
The alternating letters of 'onwall' are 'OWL'.
(Other definitions for owl that I've seen before include "eg Snowy, Tawny" , "Fly low like a bird" , "Mainly nocturnal bird" , "Bird of prey with front-facing eyes" , "Night person" .)