My regularly acerbic major (6)
I believe the answer is:
crikey
'my' is the definition.
(both are expressions of surprise)
'regularly acerbic major' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'major' becomes 'key'.
The alternating letters of 'acerbic' are 'cri'.
'cri'+'key'='CRIKEY'
(Other definitions for crikey that I've seen before include "Good heavens!" , "Cor blimey!" , "by George" , "Gosh!" , "Good Lord!" .)