Soup in mug taken up by barman is hot (6)
I believe the answer is:
borsch
'soup' is the definition.
(borsch is a kind of soup)
'mug taken up by barman is hot' is the wordplay.
'mug' becomes 'rob' (I've seen this in other clues**).
'taken up' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'barman' becomes 'sc' (I've seen this in other clues).
'is hot' becomes 'h'.
'rob' reversed gives 'bor'.
'bor'+'sc'+'h'='BORSCH'
'in' is the link.
(Other definitions for borsch that I've seen before include "Russian or Polish soup usually containing beet juice" , "Russian beetroot soup" , "Soup made with beetroot" .)