Holding it together after upset is what fellow’s about (8)
I believe the answer is:
cohesive
'holding it together' is the definition.
(cohering or holding together)
'after upset is what fellow's about' is the wordplay.
'after upset' says the letters should be written in reverse (in a down clue, letters go up).
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'fellow' becomes 'cove' (British slang term).
'about' indicates putting letters inside.
'is'+'eh'='iseh'
'iseh' written backwards gives 'hesi'.
'hesi' put into 'cove' is 'COHESIVE'.
(Other definitions for cohesive that I've seen before include "United" , "Tending to stick together" , "Sticking together" , "All together, holding" .)