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 the answer is hidden within the clue.
'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" .)