Chief of Staff leaves function (3)
I believe the answer is:
cos
I believe this is a triple definition!
'chief of staff' is the first definition.
(I've seen this before)
'leaves' is the second definition.
(cos lettuce leaves)
'function' is the third definition.
(mathematical function)
(Other definitions for cos that I've seen before include "Salad plant" , "providing unsatisfactory explanation?" , "Casual as" , "Lettuce with long crisp leaves" , "A long-leaved lettuce" .)