Southern tough injecting MDMA and horse (5)
I believe the answer is:
steed
'horse' is the definition.
(I know that steed is a type of warhorse)
'southern tough injecting mdma' is the wordplay.
'southern' becomes 's' (abbreviation).
'tough' becomes 'ted' (Ted is an example).
'injecting' is an insertion indicator.
'mdma' becomes 'E' (MDMA is ecstasy, ie 'E').
'ted' going around 'e' is 'teed'.
's'+'teed'='STEED'
'and' acts as a link.
(Other definitions for steed that I've seen before include "(Poetic) horse" , "Powerful horse" , "Horse, mount" , "ride" , "A mount (poet.)" .)