Left one grievance about opinion (6)
I believe the answer is:
belief
'opinion' is the definition.
(an opinion is a personal belief)
'left one grievance about' is the wordplay.
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'grievance' becomes 'beef' (I've seen this in another clue).
'about' is an insertion indicator.
'l'+'i'='li'
'li' going inside 'beef' is 'BELIEF'.
(Other definitions for belief that I've seen before include "Faith, credence" , "Persuasion" , "Faith, doctrine" , "Principle accepted without proof" , "Firm conviction" .)