Sprout upsetting a regiment (9)
I believe the answer is:
germinate
'sprout' is the definition.
(I know that sprout can be written as germinate)
'upsetting a regiment' is the wordplay.
'upsetting' indicates an anagram.
'a'+'regiment'='aregiment'
'aregiment' anagrammed gives 'GERMINATE'.
(Other definitions for germinate that I've seen before include "Sprout as in a regiment" , "(Of seed) start to shoot" , "Put forth shoots, develop and grow" , "Mere giant (anag) - start to grow" , "Sprout - develop" .)