Chap in a mix-up (3)
I believe the answer is:
ian
'chap' is the definition.
(man's name)
'in a mix-up' is the wordplay.
'mix-up' indicates anagramming the letters.
'in'+'a'='ina'
'ina' is an anagram of 'IAN'.
(Other definitions for ian that I've seen before include "Botham or Chappell?" , "- McEwan, Eng. writer" , "Male" , "Scotsman?" , "Sir - McKellen, actor" .)