Master reacts offensively principally falling over rootstock (5)
I believe the answer is:
orris
'rootstock' is the definition.
(preparation of iris rootstock)
'master reacts offensively principally falling over' is the wordplay.
'master' becomes 'sir' (a schoolmaster may be addressed as sir).
'principally' suggests taking the first letters.
'falling over' is a reversal indicator (letters falling back).
The first letters of 'reacts offensively' is 'ro'.
'sir'+'ro'='sirro'
'sirro' backwards is 'ORRIS'.
(Other definitions for orris that I've seen before include "Perfume root" , "Fragrant rootstock" , "Plant rhizome used in perfumery" , "perfumery ingredient" , "Fragrant root used in perfumery" .)