Rank “two star”, being most pessimistic (2,5)
I believe the answer is:
at worst
'being most pessimistic' is the definition.
(in the most pessimistic scenario)
'rank two star' is the wordplay.
'rank' indicates anagramming the letters.
'two'+'star'='twostar'
'twostar' with letters rearranged gives 'AT WORST'.
(Other definitions for at worst that I've seen before include "most pessimistically" , "Taking the most pessimistic view" , "In the most negative circumstances" , "Taking blackest view" , "in the most serious state" .)