The sun now is low (3)
I believe the answer is:
sad
'low' is the definition.
(I've seen this before)
'the sun now' is the wordplay.
'the sun' becomes 'S' (abbreviation).
'now' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
's'+'ad'='SAD'
'is' acts as a link.
(Other definitions for sad that I've seen before include "Blue" , "Tragic" , "Despondent" , "Gloomy" , "Mournful and depressed" .)