Monkey to pass no sub up (7)
I believe the answer is:
colobus
'monkey' is the definition.
(I know that colobus is a type of Old World monkey)
'pass no sub up' is the wordplay.
'pass' becomes 'col' (col is a term for a mountain pass).
'no' becomes 'O' (O resembles 0).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'sub' backwards is 'bus'.
'col'+'o'+'bus'='COLOBUS'
'to' is the link.
(Other definitions for colobus that I've seen before include "Seeing monkey" , "Slender monkey" , "Type of monkey" , "Leaf-eating monkey" , "African monkey" .)