Describing the athlete as one kept in by a broken leg (5)
I believe the answer is:
agile
'describing the athlete as' is the definition.
I can't tell whether this definition defines the answer.
'one kept in by a broken leg' is the wordplay.
'one' becomes 'i' (Roman numeral).
'kept in' is an insertion indicator.
'by' says to put letters next to each other.
'broken' indicates an anagram.
'leg' is an anagram of '[leg]'.
'a' put next to '[leg]' is 'a[leg]'.
'i' put within 'agle' is 'AGILE'.
Can you help me to learn more?
(Other definitions for agile that I've seen before include "Able to move quickly and easily" , "Moving quickly/easily" , "athletic" , "Nimble, lively" , "Sprightly" .)