An exclamation of "Oh", Charlie admitted (3)
I believe the answer is:
och
'an exclamation' is the definition.
(I have seen 'Scots exclamation ' mean 'och' so perhaps 'exclamation' could also mean 'och')
'oh charlie admitted' is the wordplay.
'charlie' becomes 'C' (phonetic alphabet).
'admitted' means one lot of letters goes inside another (inserted letters are admitted in).
'oh' placed around 'c' is 'OCH'.
'of' acts as a link.
(Other definitions for och that I've seen before include "Scots exclamation" , "Scotsman may cry" , "Scottish exclamation" , "Scots expression of impatience" , "Jock's expression" .)