I shake a leg about being nimble (5)
I believe the answer is:
agile
'nimble' is the definition.
(agile can mean nimble or athletic)
'i shake a leg about' is the wordplay.
'shake' is an anagram indicator.
'about' indicates putting letters inside.
'a'+'leg'='aleg'
'aleg' with letters rearranged gives 'agle'.
'i' going into 'agle' is 'AGILE'.
'being' acts as a link.
(Other definitions for agile that I've seen before include "Nimble, quick-moving" , "Lithely moving" , "Nimble, lively" , "Nimble, fleet" , "athletic" .)