Employ fusileer, not flier (3)
I believe the answer is:
use
'employ' is the definition.
(I know that employ can be written as use)
'fusileer not flier' is the wordplay.
'not' indicates named letters should be taken away.
'fusileer' with 'flier' taken away is 'USE'.
(Other definitions for use that I've seen before include "Value" , "Application" , "Exploit unfairly" , "Benefit" , "consume; exploit" .)