One of a pair to pass by (7)
I believe the answer is:
slipper
'one of a pair' is the definition.
(I've seen this in another clue)
'pass by' is the wordplay.
'pass' becomes 'slip' (I can't justify this - if you can you should believe this answer much more).
'by' becomes 'per' (I've seen this in another clue).
'slip'+'per'='SLIPPER'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for slipper that I've seen before include "Type of footwear" , "House shoe" , "Indoor footwear" , "One losing footing" , "Shoe worn indoors" .)