A sign, primarily, is put in street to help (6)
I believe the answer is:
assist
'help' is the definition.
(assisting is a kind of helping)
'a sign primarily is put in street' is the wordplay.
'primarily' says to take the initial letters.
'put in' is an insertion indicator.
'street' becomes 'st'.
The first letter of 'sign' is 's'.
'is' placed within 'st' is 'sist'.
'a'+'s'+'sist'='ASSIST'
'to' is the link.
(Other definitions for assist that I've seen before include "Pass to scorer" , "provide backing" , "Understand by instinct" , "Work as a subordinate" , "Give a hand to" .)