Suitable trial for Darwin's survivors (7)
I believe the answer is:
fittest
'darwin's survivors' is the definition.
(Charles Darwin, survival of the fittest)
'suitable trial' is the wordplay.
'suitable' becomes 'fit' (similar in meaning).
'trial' becomes 'test' (synonyms).
'fit'+'test'='FITTEST'
'for' acts as a link.
(Other definitions for fittest that I've seen before include "'Survival of the - - '" , "superlatively healthy" , "Most suitable" , "most sound" , "Healthiest" .)