Kind of poppy composition starts off in upbeat medley (5)
I believe the answer is:
opium
'kind of poppy' is the definition.
(I've seen this in another clue)
'composition starts off in upbeat medley' is the wordplay.
'composition' becomes 'op' (I've seen this in other clues).
'starts off' suggests taking the first letters.
The first letters of 'in upbeat medley' is 'ium'.
'op'+'ium'='OPIUM'
(Other definitions for opium that I've seen before include "where horse originally came from?" , "Narcotic derived from poppies" , "wars with China" , "Narcotic drug" , "justification for war once?" .)