My boss steals flipping silver? Blimey! (4)
I believe the answer is:
egad
'blimey' is the definition.
(both are expressions of surprise)
'my boss steals flipping silver?' is the wordplay.
'my boss' becomes 'ed' (abbreviation for editor, the setter's boss).
'steals' indicates putting letters inside.
'flipping' says the letters should be written in reverse.
'silver?' becomes 'Ag' (Ag is the chemical symbol for silver).
'ag' in reverse letter order is 'ga'.
'ed' placed around 'ga' is 'EGAD'.
(Other definitions for egad that I've seen before include "My word once" , "Old exclamation; aged (anagram)" , "traditional oath" , "Mild oath - aged (anag)" , "Oath sworn" .)