Upset about argument involving brothers essentially (9)
I believe the answer is:
overthrow
'upset' is the definition.
(I know that upset can be written as overthrow)
'about argument involving brothers essentially' is the wordplay.
'about' becomes 'over' (both can mean around).
'argument' becomes 'row' (to row is to argue).
'involving' is an insertion indicator.
'essentially' says to take the centre.
The central letters of 'brothers' are 'th'.
'over'+'row'='overrow'
'overrow' placed around 'th' is 'OVERTHROW'.
(Other definitions for overthrow that I've seen before include "Extra run perhaps" , "Bring down (government)" , "Fall" , "caused discomfiture" , "Error on cricket field - defeat" .)