Capital point about vial's content (5)
I believe the answer is:
sofia
'capital' is the definition.
(capital of Bulgaria)
'point about vial's content' is the wordplay.
'point' becomes 'S' (South).
'about' becomes 'of' ().
'content' means to look at the middle letters.
The centre of 'vial' is 'ia'.
's'+'of'+'ia'='SOFIA'
(Other definitions for sofia that I've seen before include "capital city" , "seat of government" , "Foreign city" , "Bulgarian capital" , "Bulgarian city" .)