Endlessly queue up for nothing (3)
I believe the answer is:
nil
'nothing' is the definition.
(eg nil is zero in football)
'endlessly queue up' is the wordplay.
'endlessly' means to remove the last letter.
'queue' becomes 'line' (a line of people).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'line' with its last letter removed is 'lin'.
'lin' back-to-front is 'NIL'.
'for' is the link.
(Other definitions for nil that I've seen before include "Don't despair - . . . desperandum" , "Latin zero" , "duck" , "Non-existent" , "Nothing, no value" .)