Sign up to follow space command (6)
I believe the answer is:
enjoin
'command' is the definition.
(thesaurus)
'sign up to follow space' is the wordplay.
'sign up' becomes 'join' (I've seen this before).
'to follow' is a charade indicator (letters next to each other).
'space' becomes 'en'.
'join' after 'en' is 'ENJOIN'.
(Other definitions for enjoin that I've seen before include "Place an injunction on" , "Impose an order upon" , "Instruct (to do a thing)" , "Advise" , "Prohibit by official injunction" .)