Not out, with umpire dour but ultimately not reacting (5)
I believe the answer is:
inert
'not reacting' is the definition.
(I've seen this in another clue)
'not out with umpire dour but ultimately' is the wordplay.
'not out' becomes 'in' ('in' is the opposite to 'out').
'with' means one lot of letters go next to another.
'ultimately' indicates one should take the final letters.
The last letters of 'umpire dour but' are 'ert'.
'in'+'ert'='INERT'
(Other definitions for inert that I've seen before include "Leaden" , "Chem. non-reactive" , "Slow and apathetic" , "Not active or reactive" , "Unmoving" .)