Most important chap concealing ego (4)
I believe the answer is:
main
'most important' is the definition.
(I've seen this before)
'chap concealing ego' is the wordplay.
'chap' becomes 'man' (I've seen this before**).
'concealing' is an insertion indicator.
'ego' becomes 'i'.
'man' placed around 'i' is 'MAIN'.
(Other definitions for main that I've seen before include "Salt water" , "river in Germany" , "Cardinal" , "Might on the high seas" , "Ocean or principal pipe" .)