Nimble footed fullback split trotters (5)
I believe the answer is:
fleet
'nimble footed' is the definition.
(thesaurus)
'fullback split trotters' is the wordplay.
'fullback' becomes 'l' (I can't justify this - if you can you should give a lot more credence to this answer).
'split' is an insertion indicator (some letters split a word into two).
'trotters' becomes 'feet' (trotters are pigs' feet).
'l' inserted inside 'feet' is 'FLEET'.
Can you help me to learn more?
(Other definitions for fleet that I've seen before include "Fast - naval force" , "Number of vehicles" , "good turn of speed" , "Ships - quick!" , "Swift; armada" .)