An exclamation from Sir Galahad (3)
I believe the answer is:
aha
'an exclamation' is the definition.
(I have seen 'Exclamation of discovery ' mean 'aha' so perhaps 'exclamation' could also mean 'aha')
'from sir galahad' is the wordplay.
'from' indicates a hidden word.
'AHA' is hidden within 'sir galahad'.
(Other definitions for aha that I've seen before include "Exclamation of triumph or surprise, a palindrome" , "it expresses surprise" , "I get it!" , "I'm surprised" , "Expression of surprise, satisfaction or triumph" .)