Medicines that vary in cost (6)
I believe the answer is:
tonics
'medicines' is the definition.
(tonic is a kind of medicine)
'vary in cost' is the wordplay.
'vary' indicates an anagram.
'in'+'cost'='incost'
'incost' with letters rearranged gives 'TONICS'.
'that' is the link.
(Other definitions for tonics that I've seen before include "Keys" , "Pick-me-ups" , "Potions to strengthen and invigorate" , "contents of medicine cupboards" , "Medicines or drinks with invigorating effects" .)