When the sun crosses the equator (7)
I believe the answer is:
equinox
'when the sun crosses the equator' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for equinox that I've seen before include "Day with 12 hours of light" , "Vernal/autumnal --" , "Feature of spring" , "When night=day in length" , "Time of equal day and night length" .)