Bird's circuit over part of mansion (7)
I believe the answer is:
lapwing
'bird's' is the definition.
(lapwing is a kind of bird)
'circuit over part of mansion' is the wordplay.
'circuit' becomes 'lap' (synonyms).
'over' says to put letters next to each other.
'part of mansion' becomes 'wing' (a wing is one side of a large building).
'lap'+'wing'='LAPWING'
(Other definitions for lapwing that I've seen before include "Green plover" , "Large plover with a loud call -- peewit" , "Type of plover" , "wader" , "Plover, pewit" .)