Treat sprain with supplement of root vegetables (8)
I believe the answer is:
parsnips
'root vegetables' is the definition.
(parsnip is a kind of root vegetable)
'treat sprain with supplement' is the wordplay.
'treat' indicates an anagram.
'with' means one lot of letters go next to another.
'supplement' becomes 'ps' (I can't explain this - if you can you should give a lot more credence to this answer).
'sprain' is an anagram of 'parsni'.
'parsni'+'ps'='PARSNIPS'
'of' is the link.
Can you help me to learn more?
(Other definitions for parsnips that I've seen before include "Spin spar around the vegetables" , "Fine words butter no -- (proverb)" , "not enhanced by fine words?" , "Things growing in garden" , "roots" .)