Reason given from French and Italian leader (6)
I believe the answer is:
deduce
'reason' is the definition.
(deducing is a kind of reasoning)
'from french and italian leader' is the wordplay.
'from french' becomes 'de' (French for 'from').
'and' means one lot of letters go next to another.
'italian leader' becomes 'duce' (I've seen this before).
'de'+'duce'='DEDUCE'
'given' acts as a link.
(Other definitions for deduce that I've seen before include "Figure out as Sherlock did" , "Arrive at conclusion through reasoning, as Sherlock Holmes did" , "Infer from the evidence" , "Infer like Sherlock" , "Infer by reasoning" .)