Brood over party leader over in English county (7)
I believe the answer is:
suffolk
'english county' is the definition.
(I've seen this before)
'brood over party leader over in' is the wordplay.
'brood' becomes 'sulk' ('sulk' can be a synonym of 'brood').
'over party leader' becomes 'off' (I am not sure about this - if you are sure you should believe this answer much more).
'over' shows that the letters should be reversed in order.
'in' is an insertion indicator.
'off' reversed gives 'ffo'.
'sulk' enclosing 'ffo' is 'SUFFOLK'.
Can you help me to learn more?
(Other definitions for suffolk that I've seen before include "East Anglian county" , "English county" , "Eastern county" , "Ipswich's county" , "Part of East Anglia" .)