Couple bitten by mad rats given dreadful prognosis (2,5)
I believe the answer is:
at worst
'couple bitten by mad rats given dreadful prognosis' is the definition.
I can't judge whether this definition defines the answer.
'couple bitten by mad rats' is the wordplay.
'couple' becomes 'two' (two people or things).
'bitten by' indicates putting letters inside (I've seen this in other clues).
'mad' indicates an anagram.
'rats' anagrammed gives 'arst'.
'two' put within 'arst' is 'AT WORST'.
Can you help me to learn more?
(Other definitions for at worst that I've seen before include "most pessimistically" , "Taking the most pessimistic view" , "Taking blackest view" , "in the most serious state" , "In the most negative circumstances" .)