A hard answer I suddenly see (3)
I believe the answer is:
aha
'i suddenly see' is the definition.
(exclamation meaning 'I see!')
'a hard answer' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'answer' becomes 'a' (abbreviation eg in Q and A).
'a'+'h'+'a'='AHA'
(Other definitions for aha that I've seen before include "Bingo" , "it expresses surprise" , "I've got it!" , "Exclamation of triumph or surprise, a palindrome" , "I'm surprised" .)