Defeat leader of hecklers in public spat (9)
I believe the answer is:
overthrow
'defeat' is the definition.
(I've seen this before)
'leader of hecklers in public spat' is the wordplay.
'leader of' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'public' becomes 'overt' ('overt' is associated in meaning with 'public').
'spat' becomes 'row'.
The initial letter of 'hecklers' is 'h'.
'overt'+'row'='overtrow'
'h' going inside 'overtrow' is 'OVERTHROW'.
(Other definitions for overthrow that I've seen before include "Worst" , "Vanquish" , "Unexpected run" , "Error on cricket field - defeat" , "Bring down a ruler" .)