Monitor head of welterweight boxer, perhaps after disturbing chat (8)
I believe the answer is:
watchdog
'monitor' is the definition.
(thesaurus)
'head of welterweight boxer perhaps after disturbing chat' is the wordplay.
'head of' suggests taking the first letters.
'boxer perhaps' becomes 'dog' (boxer is a kind of dog).
'after' is a charade indicator (letters next to each other).
'disturbing' is an anagram indicator (I've seen this in other clues).
The initial letter of 'welterweight' is 'w'.
'chat' with letters rearranged gives 'atch'.
'dog' after 'atch' is 'atchdog'.
'w'+'atchdog'='WATCHDOG'
(Other definitions for watchdog that I've seen before include "Monitoring body" , "Ombudsman" , "Monitoring group" , "TV consumer programme" , "protector" .)