Nice lemon in tonic, mixed right inside (6)
I believe the answer is:
citron
'nice lemon' is the definition.
('lemon' in French - Nice is in France)
'tonic mixed right inside' is the wordplay.
'mixed' is an anagram indicator.
'right' becomes 'r' (common abbreviation).
'inside' indicates putting letters inside.
'tonic' is an anagram of 'citon'.
'citon' enclosing 'r' is 'CITRON'.
'in' is the link.
(Other definitions for citron that I've seen before include "Large lemon-like fruit" , "It's like a lemon" , "Tree producing fruit" .)