Suitability (3)
'suitability' is the definition.
(I know that fitness is a type of suitability)
This is all the clue.
(Other definitions for fitness that I've seen before include "Suitability - health" , "State of good health" , "condition" , "Competence" , "Well-being" .)
(Other definitions for fit that I've seen before include "bout?" , "Match" , "Suitable; in good health" , "Of suitable quality" , "Sudden attack,of coughing say" .)