Baseball teams born in Estonia, content (5)
I believe the answer is:
nines
'baseball teams' is the definition.
('nine' can be a synonym of 'baseball team')
'born in estonia content' is the wordplay.
'content' says the answer is hidden in the clue (I've seen 'contents of' mean this).
'NINES' can be found hidden inside 'born in estonia'.
(Other definitions for nines that I've seen before include "squares" , "Dressed to these is to be perfectly turned out" , "Numbers to which one may be dressed up?" , "In emergency, press three of these" , "Dressed to these and you're well-turned-out" .)