Seconds not yet composed for final stage (6)
I believe the answer is:
sunset
'final stage' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'seconds not yet composed' is the wordplay.
'seconds' becomes 's' (abbreviation).
'not' becomes 'un' (prefix meaning 'not').
'yet composed' becomes 'set' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
's'+'un'+'set'='SUNSET'
'for' is the link.
Can you help me to learn more?
(Other definitions for sunset that I've seen before include "Time of day when the light begins to fail" , "Twilight time" , "The fading light of evening" , "Musical from film about Hollywood" , "After which day becomes night" .)