Diplomatic agent first collared by underground worker (8)
I believe the answer is:
minister
'diplomatic agent' is the definition.
(thesaurus)
'first collared by underground worker' is the wordplay.
'first' becomes 'ist' (ist resembles 1st).
'collared by' is an insertion indicator (I've seen this in other clues).
'underground worker' becomes 'miner' (someone working underground in a mine).
'ist' placed inside 'miner' is 'MINISTER'.
(Other definitions for minister that I've seen before include "Holder of office in government or church" , "Parson" , "Merit sin (anag.)" , "Clergyman - politician" , "Head of a government department" .)