Lab experimented with a tonic of plants (9)
I believe the answer is:
botanical
'of plants' is the definition.
(I've seen this before)
'lab experimented with a tonic' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for botanical that I've seen before include "(Gardens) of scientific interest" , "Relating to the study of plants" , "Description of some gardens" .)