A number on side street taking the most negative view (2,5)
I believe the answer is:
Here is my best explanation:
'taking the most negative view' is the definition.
'a number on side street' is the subsidiary indication.
'number' becomes 'two' (two is a kind of number**).
'on' means one lot of letters go next to another.
'side' becomes 'r' (abbreviation for right).
'street' becomes 'st'.
(Other definitions for at worst that I've seen before include "most pessimistically", "Taking the most pessimistic view", "In the most negative circumstances", "in the most serious state", "To be pessimistic" and "Taking blackest view".)