Impose fine on illegal street trader first of all (5)
I believe the answer is:
foist
'impose' is the definition.
(I know that foist is a more specific form of impose)
'fine on illegal street trader first of all' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'first of all' suggests taking the first letters.
The initial letters of 'on illegal street trader' is 'oist'.
'f'+'oist'='FOIST'
(Other definitions for foist that I've seen before include "Impose, inflict" , "Insinuate" , "Impose (upon)" , "palm off" , "Pass off, impose (on)" .)