Fathead occasionally ignored, I see (3)
I believe the answer is:
aha
'i see' is the definition.
(both are said after realising something)
'fathead occasionally ignored' is the wordplay.
'occasionally ignored' indicates alternate letters (ignore occasional letters and keep the rest).
The alternating letters of 'fathead' are 'AHA'.
(Other definitions for aha that I've seen before include "I've got it!" , "Triumphant cry" , "it expresses surprise" , "Cry of triumph" , "Exclamation of triumph or mockery, a palindrome" .)