Carreras cut in half for ages (4)
I believe the answer is:
eras
'for ages' is the definition.
(I've seen this in another clue)
'carreras cut in half' is the wordplay.
'cut in half' suggests halving the letters.
'carreras' cut in half is 'ERAS'.
(Other definitions for eras that I've seen before include "Periods of time" , "Epochs" , "typeface" , "Periods, ages" , "Long and distinct periods of history" .)