Farm building -- excellent -- housing large goose (8)
I believe the answer is:
barnacle
'goose' is the definition.
(barnacle goose is a kind of goose)
'farm building excellent housing large' is the wordplay.
'farm building' becomes 'barn' (barn is a kind of farm building).
'excellent' becomes 'ace' (ace can informally mean very good).
'housing' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'ace' going around 'l' is 'acle'.
'barn'+'acle'='BARNACLE'
(Other definitions for barnacle that I've seen before include "Crustacean that sticks to rocks etc" , "Marine crustacean - European goose" , "One that sticks" , "A shellfish that sticks to hulls etc" , "Clinging sort" .)