One medium paint used around percussion instrument (7)
I believe the answer is:
timpani
'percussion instrument' is the definition.
(I know that Timpani is a type of percussion instrument)
'one medium paint used' is the wordplay.
'one' becomes 'I' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'used' indicates anagramming the letters.
'i'+'m'+'paint'='impaint'
'impaint' is an anagram of 'TIMPANI'.
'around' acts as a link.
(Other definitions for timpani that I've seen before include "Those beaten" , "orchestra members" , "Tina Pim has a set of kettle drums" , "Features of kitchen" , "> kitchen equipment?" .)