Commanding officer's lettuce (3)
I believe the answer is:
cos
This could be a double definition.
'commanding officer's' is the first definition.
'co's' can be an answer for 'officer's' (Commanding Officer abbreviation). I am not certain of the 'commanding' bit.
'lettuce' is the second definition.
(cos is a kind of lettuce)
Can you help me to learn more?
(Other definitions for cos that I've seen before include "Greek island and type of lettuce" , "A long-leaved lettuce" , "Lettuce with long crisp leaves" , "Salad ingredient" , "Chief of Staff" .)