One who habitually drinks at home, without going outside (4)
I believe the answer is:
wino
'one who habitually drinks' is the definition.
(someone who often drinks wine)
'home without going outside' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'without' becomes 'wo' (abbreviation of 'without', normally with a slash).
'going outside' means one lot of letters goes inside another (the word goes on the outside of inserted letters).
'in' placed within 'wo' is 'WINO'.
'at' acts as a link.
(Other definitions for wino that I've seen before include "Down-and-out drunkard" , "As a dipso" , "Toper" , "Street drunkard" , "heavy drinker" .)