They are outnumbered in play (4)
I believe the answer is:
dice
'they are' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'outnumbered in play' is the wordplay.
'outnumbered' becomes 'c' (I am not sure about this - if you are sure you should believe this answer much more).
'in' is an insertion indicator.
'play' becomes 'die' (dying is a kind of playing).
'c' going within 'die' is 'DICE'.
Can you help me to learn more?
(Other definitions for dice that I've seen before include "Take a risk" , "Random number generators" , "Gaming cube(s)" , "Cubes used in gambling" , "Cut into small cubes" .)