A quick haircut outside university hall (6)
I believe the answer is:
atrium
'hall' is the definition.
(I've seen this before)
'a quick haircut outside university' is the wordplay.
'quick haircut' becomes 'trim' (I've seen this in another clue).
'outside' means one lot of letters goes inside another (some letters go outside others).
'university' becomes 'U' (abbreviation as in UMass).
'trim' going around 'u' is 'trium'.
'a'+'trium'='ATRIUM'
(Other definitions for atrium that I've seen before include "WW1 battleground" , "Entrance hall" , "Central area in a building, open or with a glass roof" , "An upper chamber of the heart" , "Central chamber in an ancient Roman house" .)