You can see one on a billiard ball (4)
I believe the answer is:
spot
'see one on a billiard ball' is the definition.
'spot' can be an answer for 'see' (to spot is to notice or catch sight of). I am unsure of the 'one on a billiard ball' bit.
'you can' is the wordplay.
'you' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'can' becomes 'pot' (synonyms).
's'+'pot'='SPOT'
Can you help me to learn more?
(Other definitions for spot that I've seen before include "make out" , "Informally, catch sight of" , "Notice; sign of acne" , "Mark - position - perceive" , "See; place" .)