Complaint, large one, included confidence (6)
I believe the answer is:
belief
'confidence' is the definition.
(I've seen this before)
'complaint large one included' is the wordplay.
'complaint' becomes 'beef' (beef can mean a complaint or objection).
'large' becomes 'L' (eg in clothes sizes).
'one' becomes 'i' (Roman numeral).
'included' indicates putting letters inside.
'l'+'i'='li'
'beef' placed around 'li' is 'BELIEF'.
(Other definitions for belief that I've seen before include "Tenet, creed" , "Faith, doctrine" , "Conviction, faith" , "Creed or opinion" , "Principle accepted without proof" .)