Sleep for cadets and trainees not always possible, to begin with (6)
I believe the answer is:
catnap
'sleep' is the definition.
(I know that catnap is a more specific form of the action sleep)
'cadets and trainees not always possible to begin with' is the wordplay.
'to begin with' says to take the initial letters.
The initial letters of 'cadets and trainees not always possible' is 'CATNAP'.
'for' acts as a link.
(Other definitions for catnap that I've seen before include "Brief time out" , "Kip" , "Drop off" , "For a rest" , "Short sleep" .)