Fat reserves not specified at the end of the day (6)
I believe the answer is:
sunset
'the end of the day' is the definition.
(I've seen this before)
'fat reserves not specified' is the wordplay.
'fat' becomes 'suet' (suet is a kind of fat).
'reserves' indicates putting letters inside.
'not specified' becomes 'ns' (this might be a standard abbreviation of which I'm unaware).
'suet' placed around 'ns' is 'SUNSET'.
'at' is the link.
Can you help me to learn more?
(Other definitions for sunset that I've seen before include "Time of evening colour" , "Dusk" , "Close of day" , "Twilight time" , "Evening glow" .)