Least desirably (2,5)
I believe the answer is:
at worst
'least desirably' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for at worst that I've seen before include "In the most negative circumstances" , "in the most serious state" , "most pessimistically" , "Taking blackest view" , "Taking the most pessimistic view" .)