Goose in farm building disturbed Alec (8)
I believe the answer is:
Here is my best explanation:
'goose' is the definition.
(barnacle is a kind of goose)
'farm building disturbed alec' is the subsidiary indication.
'farm building' becomes 'barn' (barn is a kind of farm building).
'disturbed' indicates anagramming the letters.
'alec' with letters rearranged gives 'acle'.
'in' is the link.
(Other definitions for barnacle that I've seen before include "Sea creature - lean crab (anag)", "'Marine crustacean, attaches to a boat-hull (8)'", "Marine crustacean - European goose", "Marine crustacean (attaching to rocks, ships etc.)", "A shellfish that sticks to hulls etc", "Hull resident (one who clings on)?", "Crustacean that attaches to underwater surfaces", "Crustacean that sticks to rocks etc", "One reluctant to leave" and "type of goose".)