Partly withdraw, making borders of state change and decrease (6)
I believe the answer is:
secede
'partly withdraw' is the definition.
(to secede is withdraw or break away from)
'borders of state change and decrease' is the wordplay.
'borders of' suggests removing the centre.
'state change' becomes 'smoke' (smoke is a kind of state change).
'and' says to put letters next to each other.
'decrease' becomes 'cede' (I am not sure about this - if you are sure you should believe this answer much more).
'smoke' with its centre removed is 'se'.
'se'+'cede'='SECEDE'
'making' acts as a link.
(Other definitions for secede that I've seen before include "Withdraw from a body, a nation say" , "Withdraw formally from membership of an organisation" , "Withdraw from, say organisation" , "Withdraw (politically)" , "leave union" .)