Stop egghead appearing in suit (5)
I believe the answer is:
cease
'stop' is the definition.
(I know that stop can be written as cease)
'egghead appearing in suit' is the wordplay.
'egghead' becomes 'e' (head letter of 'egg').
'appearing in' is an insertion indicator.
'suit' becomes 'case' (synonyms).
'e' going inside 'case' is 'CEASE'.
(Other definitions for cease that I've seen before include "Stop, finish" , "Political group" , "Come to an end" , "Culminate" , "Stop, give over" .)