Theoretically, capital keeps leading man in order (1,6)
I believe the answer is:
a priori
'theoretically' is the definition.
(I've seen this before)
'capital keeps leading man in order' is the wordplay.
I cannot quite understand how this works, but
'capital' could be 'rio' (I've seen this before) and 'rio' is found within the answer.
'man' could be 'r' (abbreviation for rook, a chess piece or 'man') and 'r' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for a priori that I've seen before include "Based on theory (Latin)" , "reasoned logically" , "not needing evidence" , "worked with deductions" , "'Derived by logic, without observed facts (1,6)'" .)