Hot drink for one, in Students' Union (5)
I believe the answer is:
negus
'hot drink' is the definition.
(I've seen this before)
'for one in students' union' is the wordplay.
'for one' becomes 'eg' (both can mean 'for example').
'in' means one lot of letters goes inside another.
'students' union' becomes 'NUS' (National Union of Students).
'eg' inserted into 'nus' is 'NEGUS'.
(Other definitions for negus that I've seen before include "Hot fortified drink" , "Ruler in Africa" , "Hot wine" , "Hot stimulant" , "Abyssinian king - spiced drink" .)