Fighting earlier involving leaders in Glorious Revolution (5)
I believe the answer is:
aggro
'fighting' is the definition.
(I've seen this before)
'earlier involving leaders in glorious revolution' is the wordplay.
'earlier' becomes 'ago' (I've seen this before).
'involving' is an insertion indicator.
'leaders in' indicates taking the first letters.
The first letters of 'glorious revolution' is 'gr'.
'ago' enclosing 'gr' is 'AGGRO'.
(Other definitions for aggro that I've seen before include "Grief" , "belligerent behaviour" , "Trouble; forceful behaviour" , "Hostility" , "Provocation (informal)" .)