Dodgy challenge to get fit (12)
I believe the answer is:
questionable
'dodgy' is the definition.
'challenge to get fit' is the wordplay.
'challenge' becomes 'question' (questioning is a kind of challenging**).
'to get' says to put letters next to each other.
'fit' becomes 'able' ('able' can be similar in meaning to 'fit').
'question'+'able'='QUESTIONABLE'
(Other definitions for questionable that I've seen before include "Iffy" , "Suspect, unproven" , "Doubtful - possibly dishonest" , "Uncertain" , "Dubious" .)