One’s in large part agile (7)
I believe the answer is:
lissome
'agile' is the definition.
(I've seen this in another clue)
'one's in large part' is the wordplay.
'one's' becomes 'i's' (Roman numeral).
'in' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'part' becomes 'some' (I've seen this before).
'l'+'some'='lsome'
'is' placed within 'lsome' is 'LISSOME'.
(Other definitions for lissome that I've seen before include "Moving supply" , "So smile, being agile and lithe" , "Willowy" , "Slim, supple, graceful" , "Thin - supple - graceful" .)