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 "Marine crustacean (attaching to rocks, ships etc.)", "Sea creature - lean crab (anag)", "Crustacean attached to rocks", "A shellfish that sticks to hulls etc", "Hull resident (one who clings on)?", "Be carnal consuming this shellfish", "'Marine crustacean, attaches to a boat-hull (8)'", "Crustacean that adheres to ships' bottom", "Crustacean that attaches to submerged surfaces" and "Marine crustacean - European goose".)