Helps foolish person -- is crossing street (7)
I believe the answer is:
assists
'helps' is the definition.
(assisting is a kind of helping)
'foolish person is crossing street' is the wordplay.
'foolish person' becomes 'ass' (I've seen this before**).
'crossing' is an insertion indicator.
'street' becomes 'st'.
'is' placed around 'st' is 'ists'.
'ass'+'ists'='ASSISTS'
(Other definitions for assists that I've seen before include "Lends a hand" , "Provides" , "Aids" , "Contributes to success" , "Helps or waits on" .)