The chief chap, I introduced (4)
I believe the answer is:
main
'the chief' is the definition.
(I've seen this in another clue)
'chap i introduced' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'introduced' is an insertion indicator.
'man' enclosing 'i' is 'MAIN'.
(Other definitions for main that I've seen before include "Source of water" , "Chief; sea; pipe" , "Principal or high seas" , "Foremost" , "Cardinal" .)