They say command and control (4)
I believe the answer is:
rein
'control' is the definition.
(reining is a kind of controlling)
'they say command' is the wordplay.
'they say' indicates a 'sounds like' (homophone) clue.
'command' becomes 'reign'.
'reign' is a homophone of 'REIN'.
'and' acts as a link.
(Other definitions for rein that I've seen before include "Horse's steering strap" , "Harness item" , "Long strap to restrain" , "Restraining lead" , "(Horse's) guiding strap" .)