No time for daydreaming (5)
I believe the answer is:
night
'no' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'time for daydreaming' is the wordplay.
'time' becomes 't' (abbreviation).
'for' says to put letters next to each other (I've seen this in other clues).
'daydreaming' becomes 'nigh' (I can't explain this - if you can you should give a lot more credence to this answer).
't' after 'nigh' is 'NIGHT'.
Can you help me to learn more?
(Other definitions for night that I've seen before include "''Twas the ... before Christmas, when all through the house / Not a creature was stirring, not even a mouse''" , "Time for turning" , "Ignorance" , "Dark hours" , "See 8" .)