A number on side street taking the most negative view (2,5)
I believe the answer is:
at worst
'taking the most negative view' is the definition.
'a number on side street' is the wordplay.
'number on' becomes 'two' (two is a kind of number. I am not sure about the 'on' bit.**).
'side' becomes 'r' (abbreviation for right).
'street' becomes 'st'.
'a'+'two'+'r'+'st'='AT WORST'
(Other definitions for at worst that I've seen before include "Taking the most pessimistic view" , "most pessimistically" , "Taking blackest view" , "in the most serious state" , "In the most negative circumstances" .)