Problem involving credit leading to melée (5)
I believe the answer is:
scrum
'melee' is the definition.
(both can mean a group of people fighting)
'problem involving credit' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'involving' indicates putting letters inside.
'credit' becomes 'cr'.
'sum' enclosing 'cr' is 'SCRUM'.
'leading to' acts as a link.
(Other definitions for scrum that I've seen before include "disorderly group" , "Rugby get-together" , "Disorderly mob" , "Rugby huddle" , "A rugby formation or confused struggle" .)