Knight, limber, might be more agile (7)
I believe the answer is:
nimbler
'more agile' is the definition.
(I've seen this before)
'knight limber might be' is the wordplay.
'knight' becomes 'n' (chess).
'might be' indicates an anagram.
'limber' with letters rearranged gives 'imbler'.
'n'+'imbler'='NIMBLER'
(Other definitions for nimbler that I've seen before include "Lighter moving" , "More agile" , "more alert" .)