That is to be found in side street (2,3)
I believe the answer is:
id est
'that is' is the definition.
(synonyms)
'found in side street' is the wordplay.
'found in' says the answer is hidden in the clue.
'ID EST' can be found hidden inside 'side street'.
'to be' is the link.
(Other definitions for id est that I've seen before include "ie (L)" , "I mean to say!" , "In other words" , "That is to say (Latin)" , "Ie, in full" .)