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' means one lot of letters goes inside another.
'not specified' becomes 'ns' (this could be a standard abbreviation of which I'm unaware).
'suet' enclosing 'ns' is 'SUNSET'.
'at' acts as a 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" .)