Fall behind it at the end of the day (5)
I believe the answer is:
night
'the end of the day' is the definition.
(night follows day)
'fall behind it' is the wordplay.
'fall' becomes 'nigh' (I can't explain this - if you can you should give a lot more credence to this answer).
'behind' says to put letters next to each other.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'nigh'+'t'='NIGHT'
'at' is the link.
Can you help me to learn more?
(Other definitions for night that I've seen before include "Hours of darkness" , "''Twas the ... before Christmas, when all through the house / Not a creature was stirring, not even a mouse''" , "Tennyson's "black bat"" , "Dark hours" , "Darkness; see you in the morning" .)