Sobriquet of English ambassador entertained by former PM (7)
I believe the answer is:
epithet
'sobriquet' is the definition.
(I've seen this before)
'english ambassador entertained by former pm' is the wordplay.
'english' becomes 'e' (abbreviation).
'ambassador' becomes 'he' (His or Her Excellency).
'entertained by' is an insertion indicator (I've seen this in other clues).
'former pm' becomes 'pitt' (I've seen this before).
'he' inserted within 'pitt' is 'pithet'.
'e'+'pithet'='EPITHET'
'of' is the link.
(Other definitions for epithet that I've seen before include "Appellation" , "Descriptive work or phrase" , "Phrase expressing some quality of a person or thing" , "Fitting description" , "Descriptive word or phrase added to a name" .)