Those expected to survive are ready to try (7)
I believe the answer is:
'those expected to survive' is the definition.
I can't judge whether this defines the answer.
'ready to try' is the wordplay.
'ready' becomes 'fit' (I've seen this in another clue).
'to try' becomes 'test' ('test' can be a synonym of 'try').
'are' is the link.
Can you help me to learn more?
(Other definitions for fittest that I've seen before include "Healthiest" , "most sound" , "'Survival of the - - '" , "Most suitable" , "superlatively healthy" .)