Before birth of current emperor, quite regularly accepted (2,5)
I believe the answer is:
in utero
'before birth' is the definition.
(I've seen this before)
'current emperor quite regularly accepted' is the wordplay.
'current' becomes 'I' (symbol for electric current).
'emperor' becomes 'nero' (Roman emperor).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'accepted' means one lot of letters goes inside another.
The alternate letters of 'quite' are 'ut'.
'nero' enclosing 'ut' is 'nutero'.
'i'+'nutero'='IN UTERO'
'of' is the link.
(Other definitions for in utero that I've seen before include "being developed" , "Before birth (Latin)" , "Within the womb - routine (anag)" , "Embryonic" , "Where the foetus is" .)