Excellent to beset castle I supplied with soldiers, having stronger reason (1,8)
I believe the answer is:
a fortiori
'having stronger reason' is the definition.
(term in logic, from the stronger argument)
'excellent to beset castle i supplied with soldiers' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'to beset' means one lot of letters goes inside another.
'castle' becomes 'fort' (fort is a type of castle).
'supplied with' is a charade indicator (letters next to each other).
'soldiers' becomes 'OR' (Other Ranks).
'fort'+'i'+'or'='fortior'
'ai' placed around 'fortior' is 'A FORTIORI'.
(Other definitions for a fortiori that I've seen before include "By inference, even more" , "For stronger reasons" , "even more convincing reasons" , "more convincing reason" , "with better grounds" .)