Ultimately fit and lithe, trained in Air Corps (8)
I believe the answer is:
athletic
'ultimately fit and lithe trained in air corps' is the definition.
I can't tell whether this definition defines the answer.
'ultimately fit and lithe trained in air corps' is the wordplay.
'ultimately' says to take the final letters.
'and' is a charade indicator (letters next to each other).
'trained' is an anagram indicator (to train can mean to grow something into a shape).
'in' means one lot of letters goes inside another.
'air corps' becomes 'AC' (military abbreviation).
The last letter of 'fit' is 't'.
't' after 'lithe' is 'lithet'.
'lithet' with letters rearranged gives 'thleti'.
'thleti' going inside 'ac' is 'ATHLETIC'.
(Other definitions for athletic that I've seen before include "Fit and strong - football club?" , "Physically fit, strong and active" , "Vigorously active" , "Lusty" , "Strong and active, good at sports" .)