Number of clean sheets taking up most of line (3)
I believe the answer is:
nil
'number' is the definition.
(I know that nil is 0 number class)
'up most of line' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'most of' means to remove the last letter.
'line' with its final letter taken away is 'lin'.
'lin' reversed gives 'NIL'.
'of clean sheets taking' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for nil that I've seen before include "Nothing - in Latin" , "Nothing, no value" , "You get nothing" , "Latin zero" , "Love" .)