Time of longest or shortest day (8)
I believe the answer is:
solstice
'time of longest or shortest day' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for solstice that I've seen before include "When night and day are most unequal" , "it occurs biannually" , "When the sun is farthest from the celestial equator" , "Closest I come to time when sun is furthest from equator" , "Summer or winter - -" .)