Upturn in light swamping last of encircling dark (5)
I believe the answer is:
night
'dark' is the definition.
(it's dark at nighttime)
'upturn in light swamping last of encircling' is the wordplay.
'upturn in' shows that the letters should be reversed in order.
'light' becomes 'thin' (similar in meaning).
'swamping' is an insertion indicator (swamp can mean to surround or smother).
'last of' indicates one should take the final letters.
The final letter of 'encircling' is 'g'.
'thin' going around 'g' is 'thgin'.
'thgin' written backwards gives 'NIGHT'.
(Other definitions for night that I've seen before include "Time for bed" , "Saturday -- Fever (film)" , "Period of darkness" , "late" , "Time between days" .)