Listen out and sign up (6)
I believe the answer is:
Here is my best explanation:
'sign up' is the definition.
(enlisting is a kind of signing up)
'listen out' is the subsidiary indication.
'out' indicates anagramming the letters.
'listen' with letters rearranged gives 'enlist'.
'and' is the link.
(Other definitions for enlist that I've seen before include "Draft", "Listen (anag.)", "Obtain help of", "Engage", "Join the military", "Take the Queen's shilling", "augment army", "Join the armed forces", "Register" and "To join up".)