A date for one goes ahead? Blimey! (4)
I believe the answer is:
egad
'blimey' is the definition.
(both are expressions of surprise)
'a date for one goes ahead?' is the wordplay.
'a date' becomes 'ad' (AD is an example).
'for one' becomes 'eg' (I've seen this in other clues).
'goes ahead?' means one lot of letters go next to another.
'ad' put after 'eg' is 'EGAD'.
(Other definitions for egad that I've seen before include "Mild oath (archaic)" , "Oath sworn" , "Archaic exclamation of surprise" , "Archaic expression of anger, surprise, etc" , "Mild oath - aged (anag)" .)