A support that's held in extremely rebellious busts (7)
I believe the answer is:
arrests
'busts' is the definition.
(I know this)
'a support that's held in extremely rebellious' is the wordplay.
'support' becomes 'rest' (e.g. a shoulder rest is a support for shoulder).
'that's held in' is an insertion indicator.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'rebellious' with its middle taken out is 'rs'.
'rest' placed into 'rs' is 'rrests'.
'a'+'rrests'='ARRESTS'
(Other definitions for arrests that I've seen before include "has a heart attack" , "Apprehends" , "Seizes and takes into custody" , "made by police" , "Checks" .)