South-eastern people entering club in cellar (8)
I believe the answer is:
basement
'cellar' is the definition.
(both can mean the space underneath a house)
'south-eastern people entering club' is the wordplay.
'south eastern' becomes 'se' (abbreviation).
'people' becomes 'men' (men are people).
'entering' means one lot of letters goes inside another.
'club' becomes 'bat' (as in sporting equipment).
'se'+'men'='semen'
'semen' inserted inside 'bat' is 'BASEMENT'.
'in' acts as a link.
(Other definitions for basement that I've seen before include "Underground room" , "Bottom floor" , "Part of house below ground" , "Lower ground floor" , "Lowest floor of a building" .)