Stock additional tonic supply (10)
I believe the answer is:
extraction
'stock' is the definition.
(thesaurus)
'additional tonic supply' is the wordplay.
'additional' becomes 'extra' (extra means additional or surplus).
'supply' is an anagram indicator (in a supple way).
'tonic' is an anagram of 'ction'.
'extra'+'ction'='EXTRACTION'
(Other definitions for extraction that I've seen before include "breeding" , "one suffers with mouth open?" , "Removal; lineage" , "drawing" , "Ethnic origin" .)