Agile agent has just left (9)
I believe the answer is:
sprightly
'agile' is the definition.
(I've seen this before)
'agent has just left' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'has' means one lot of letters goes inside another.
'just' becomes 'right' (similar in meaning).
'left' becomes 'l' (common abbreviation).
'right'+'l'='rightl'
'spy' enclosing 'rightl' is 'SPRIGHTLY'.
(Other definitions for sprightly that I've seen before include "Lively" , "Moving nimbly" , "Active" , "Agile" , "Vivacious" .)