Some Dominicans in prison (3)
I believe the answer is:
can
'prison' is the definition.
(the can is American slang for prison)
'some dominicans in' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'CAN' is hidden within 'dominicans in'.
(Other definitions for can that I've seen before include "has power" , "John" , "Airtight metal container" , "Is able; container" , "Container (e.g. beer)" .)