High-ranking head of Inquisition appearing in Man of La Mancha? (6)
I believe the answer is:
senior
'high-ranking' is the definition.
(I've seen this before)
'head of inquisition appearing in man of la mancha?' is the wordplay.
'head of' indicates taking the first letters.
'appearing in' is an insertion indicator.
'man of la mancha?' becomes 'senor' (I've seen this in another clue).
The initial letter of 'inquisition' is 'i'.
'i' placed inside 'senor' is 'SENIOR'.
(Other definitions for senior that I've seen before include "'Older, superior (6)'" , "High in rank" , "Denoting older people" , "Superior - elder" , "Higher in rank or authority" .)