Mister, a man surprisingly normal (10)
I believe the answer is:
mainstream
'normal' is the definition.
(I've seen this before)
'mister a man surprisingly' is the wordplay.
'surprisingly' indicates an anagram.
'mister'+'a'+'man'='misteraman'
'misteraman' with letters rearranged gives 'MAINSTREAM'.
(Other definitions for mainstream that I've seen before include "Relating to what is normal or standard" , "conventional activities" , "In accordance with what is normal or standard" , "People representing most common ideas" , "type of jazz" .)