Tonic, and what are drunk with tonic by English (7)
I believe the answer is:
ginseng
'tonic' is the definition.
(I've seen this before)
'what are drunk with tonic by english' is the wordplay.
'what are drunk with tonic' becomes 'gins' (gin and tonic).
'by' is a charade indicator (letters next to each other).
'english' becomes 'eng' (abbreviation).
'gins'+'eng'='GINSENG'
'and' acts as a link.
(Other definitions for ginseng that I've seen before include "Asian plant" , "Plant tuber credited with tonic properties" , "Tuber with tonic effect" , "Medicinal tuber" , "Herbal tonic tuber" .)