Brighten up; umpire essentially excuses run blunder (9)
I believe the answer is:
refurbish
'brighten up' is the definition.
(I've seen this before)
'umpire essentially excuses run blunder' is the wordplay.
'umpire' becomes 'ref' (short for referee).
'essentially' means to look at the middle letters.
'run' becomes 'r' (cricket abbreviation).
'blunder' becomes 'bish' (I've seen this before).
The centre of 'excuses' is 'u'.
'ref'+'u'+'r'+'bish'='REFURBISH'
(Other definitions for refurbish that I've seen before include "Renovate, redecorate" , "Overhaul" , "Freshen up" , "Redecorate and brighten up" , "Clean up" .)