One narrow-minded old priest, ultimately under grown-up (5)
I believe the answer is:
bigot
'one narrow' is the definition.
(thesaurus)
'minded old priest ultimately under grown-up' is the wordplay.
'minded old' becomes 'o' (common abbreviation eg in OE for Old English. I am not sure about the 'minded' bit.).
'ultimately' suggests the final letters.
'under' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'grown-up' becomes 'big' (I've seen this in another clue).
The last letter of 'priest' is 't'.
'o'+'t'='ot'
'ot' put after 'big' is 'BIGOT'.
Can you help me to learn more?
(Other definitions for bigot that I've seen before include "Intolerantly prejudiced person" , "Intolerant type" , "Someone intolerant of opinions of others" , "One is prejudiced and intolerant" , "Deeply prejudiced and intolerant person" .)