Chap finds one with a disorder (5)
I believe the answer is:
mania
'disorder' is the definition.
(mania is a kind of disorder)
'chap finds one with a' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'finds' means one lot of letters go next to another.
'one' becomes 'I' (Roman numeral).
'with' means one lot of letters go next to another.
'man'+'i'+'a'='MANIA'
(Other definitions for mania that I've seen before include "Excessive enthusiasm" , "Over-enthusiasm" , "Craze; overexcitement" , "Frenzy" , "serious obsession" .)