Kept in the side street that is (2,3)
I believe the answer is:
id est
'that is' is the definition.
('that is' in Latin)
'kept in the side street' is the wordplay.
'kept in' indicates the answer is hidden within the clue.
'ID EST' is hidden in the letters of 'the side street'.
(Other definitions for id est that I've seen before include "That is to say (Latin)" , "ie (L)" , "Namely" , "In other words" , "that is in Rome" .)