Tackling corruption with his band, reversing poor showing in Government (7-7)
I believe the answer is:
whistle-blowing
'tackling corruption' is the definition.
(whistle-blowing is exposing corruption)
'with his band reversing poor showing in government' is the wordplay.
'with' becomes 'w' (abbreviation).
'band' becomes 'belt' (I've seen this before).
'reversing' says the letters should be written backwards.
'poor showing' becomes 'low' (low or poor quality).
'government' becomes 'g' (abbreviation for government).
'belt' written backwards gives 'tleb'.
'w'+'his'+'tleb'+'low'+'in'+'g'='WHISTLE-BLOWING'
(Another definition for whistle-blowing that I've seen is " Revealing hidden wrongdoing".)