Poles in both sides of baseball players (4)
I believe the answer is:
bars
'poles' is the definition.
'both sides of baseball players' is the wordplay.
'both sides of' says to hollow out the word (remove centre letters) (only the letters on the sides remain).
'baseball players' becomes 'batters' (batter is a kind of baseball player).
'batters' with its centre taken out is 'BARS'.
'in' is the link.
(Other definitions for bars that I've seen before include "where people get drunk" , "Prohibits; sandbanks" , "Metal rods or pubs" , "Blocks off" , "Establishments where alcoholic drinks are served" .)