Bag is a pain, being left in street (6)
I believe the answer is:
sachet
'bag' is the definition.
(sachet is a kind of bag)
'pain being left in street' is the wordplay.
'pain' becomes 'ache' (ache is a kind of pain).
'being left in' is an insertion indicator.
'street' becomes 'st'.
'ache' inserted within 'st' is 'SACHET'.
'is a' acts as a link.
(Other definitions for sachet that I've seen before include "Package" , "Small packet (of, eg, shampoo)" , "'Small soft packet, of pot-pourri perhaps (6)'" , "Cheats (anag) -- small porous bag" , "Scent bag - shampoo envelope" .)