A French farm building likely to collapse (8)
I believe the answer is:
unstable
'likely to collapse' is the definition.
(I've seen this before)
'a french farm building' is the wordplay.
'a french' becomes 'un' ('a' in French).
'farm building' becomes 'stable' (stable is a kind of farm building).
'un'+'stable'='UNSTABLE'
(Other definitions for unstable that I've seen before include "lacking security" , "Not steady, volatile" , "insecure" , "Precarious or subject to change" , "Volatile, precarious" .)