Properly, one of a pair (5)
I believe the answer is:
welly
'properly one of a pair' is the definition.
I can't tell whether this definition defines the answer.
'properly one of a pair' is the wordplay.
'properly' becomes 'well'.
'one of a pair' becomes 'y' (I can't explain this - if you can you should give a lot more credence to this answer).
'well'+'y'='WELLY'
Can you help me to learn more?
(Other definitions for welly that I've seen before include "Rubber boot (abbr)" , "Oomph" , "showing great commitment" , "Boot (informal)" , "wet-weather gear" .)