Person lingering in large building, one holding fifty (8)
I believe the answer is:
barnacle
'person lingering in large' is the definition.
I can't judge whether this defines the answer.
'building one holding fifty' is the wordplay.
'building' becomes 'barn' (barn is a kind of building).
'one' becomes 'ace' (ace has a value of one in some card games).
'holding' means one lot of letters goes inside another.
'fifty' becomes 'l' (Roman numeral).
'ace' placed around 'l' is 'acle'.
'barn'+'acle'='BARNACLE'
Can you help me to learn more?
(Other definitions for barnacle that I've seen before include "'Marine crustacean, attaches to a boat-hull (8)'" , "hanger-on" , "kind of goose" , "A shellfish that sticks to objects under water" , "Sea creature - lean crab (anag)" .)