A leading monk, upstanding character in theory (1,6)
I believe the answer is:
a priori
'in theory' is the definition.
Both the answer and definition are adverbs. Perhaps there's a link between them I don't understand?
'a leading monk upstanding character' is the wordplay.
'leading' says to put letters next to each other.
'monk' becomes 'prior' (I've seen this before).
'upstanding character' becomes 'I' (vertical or upright letter or character).
'a'+'prior'+'i'='A PRIORI'
Can you help me to learn more?
(Other definitions for a priori that I've seen before include "based on logic" , "Type of reasoning from what has gone before" , "worked with deductions" , "(Reasoning) from first principles (Latin)" , "(Of reasoning) deduction from a general principle" .)