Force trainee to execute 'duck and cover', primarily when retreating (5)
I believe the answer is:
cadet
'force trainee' is the definition.
(army cadet)
'to execute duck and cover primarily when retreating' is the wordplay.
'primarily' indicates taking the first letters.
'when retreating' says the letters should be written backwards.
The initial letters of 'to execute duck and cover' is 'tedac'.
'tedac' back-to-front is 'CADET'.
(Other definitions for cadet that I've seen before include "Officer-trainee" , "young soldier" , "not the heir" , "Trainee soldier" , "aspiring officer" .)