Where you’d have seen GP turning in referral, ominously (5)
I believe the answer is:
imola
'where you'd have seen gp' is the definition.
(city that formerly hosted the San Marino Gran Prix)
'turning in referral ominously' is the wordplay.
'turning' is a reversal indicator.
'in' says the answer is hidden in the clue.
'alomi' is hidden in the letters of 'referral ominously'.
'alomi' reversed gives 'IMOLA'.
(Other definitions for imola that I've seen before include "Grand Prix track" , "Italian motor-racing circuit" , "place for GP" , "GP's place" , "race here?" .)