Try to keep lithe, training to be so? (9)
I believe the answer is:
healthier
'try to keep lithe training to be so?' is the definition.
I can't judge whether this definition defines the answer.
'try to keep lithe training' is the wordplay.
'try' becomes 'hear' (as in to try someone in court or hear their case).
'to keep' is an insertion indicator.
'training' indicates anagramming the letters (to train can mean to grow something into a shape).
'lithe' is an anagram of 'lthie'.
'hear' going around 'lthie' is 'HEALTHIER'.
(Other definitions for healthier that I've seen before include "More in the pink" , "Fitter" , "less prone to disease" .)