No court card is popular between bridge opponents (4)
I believe the answer is:
nine
'no court card' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'popular between bridge opponents' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'between' means one lot of letters goes inside another.
'bridge opponents' becomes 'ne' (I can't justify this - if you can you should believe this answer much more).
'in' put inside 'ne' is 'NINE'.
'is' is the link.
Can you help me to learn more?
(Other definitions for nine that I've seen before include "Number of feline existences" , "So many" , "Square root of 81" , "dancing ladies?" , "Square next up after four" .)