Bird can on unstable mansard (4,6)
I believe the answer is:
sand martin
'bird' is the definition.
(sand martin is a kind of bird)
'can on unstable mansard' is the wordplay.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'on' says to put letters next to each other.
'unstable' is an anagram indicator.
'mansard' is an anagram of 'sandmar'.
'tin' after 'sandmar' is 'SAND-MARTIN'.
(Other definitions for sand martin that I've seen before include "Flyer" , "Migratory bird" , "Singer" .)