Crime relating to fight with sword (7)
I believe the answer is:
offence
'crime' is the definition.
(I know that crime can be written as offence)
'relating to fight with sword' is the wordplay.
'relating to' becomes 'of'.
'fight with sword' becomes 'fence' (fencing is a sword-fighting sport).
'of'+'fence'='OFFENCE'
(Other definitions for offence that I've seen before include "Breach of conduct" , "illegality" , "taken by one insulted" , "An act punishable by law" , "The action of attacking an enemy" .)