Agile performer ultimately taken in by agent (4)
I believe the answer is:
spry
'agile' is the definition.
('spry' can be a synonym of 'agile')
'performer ultimately taken in by agent' is the wordplay.
'ultimately' indicates one should take the final letters.
'taken in by' indicates putting letters inside.
'agent' becomes 'spy' (spy is a kind of agent).
The last letter of 'performer' is 'r'.
'r' put into 'spy' is 'SPRY'.
(Other definitions for spry that I've seen before include "(Of an old person) lively" , "'Agile, nimble (4)'" , "Fleet" , "'Active and lively, nimble (4)'" , "Nimble, brisk" .)