Hanger-on, one locking up pound by farm building (8)
I believe the answer is:
barnacle
'hanger-on' is the definition.
(thesaurus)
'one locking up pound by farm building' is the wordplay.
'one' becomes 'ace' ('ace' can be a synonym of 'one'**).
'locking up' is an insertion indicator (I've seen this in other clues).
'pound' becomes 'l'.
'by' says to put letters next to each other.
'farm building' becomes 'barn' (barn is a kind of farm building).
'ace' placed around 'l' is 'acle'.
'acle' after 'barn' is 'BARNACLE'.
(Other definitions for barnacle that I've seen before include "A shellfish that sticks to hulls etc" , "Marine crustacean - European goose" , "'Marine crustacean, attaches to a boat-hull (8)'" , "Crustacean that sticks to rocks etc" , "One reluctant to leave" .)