Insincere talk about on street? It's true! (8)
I believe the answer is:
constant
'true' is the definition.
(similar in meaning)
'insincere talk about on street?' is the wordplay.
'insincere talk' becomes 'cant' (I've seen this in another clue).
'about' is an insertion indicator.
'street?' becomes 'st' (abbreviation).
'on'+'st'='onst'
'cant' going around 'onst' is 'CONSTANT'.
'it's' acts as a link.
(Other definitions for constant that I've seen before include "Unwavering" , "Recurring and unchanging" , "Loyal - firm" , "Continuing unvarying" , "staying the same" .)