Listen out and sign up (6)
I believe the answer is:
enlist
'sign up' is the definition.
(I know that enlist is a more specific form of the action sign up)
'listen out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'listen' anagrammed gives 'ENLIST'.
'and' is the link.
(Other definitions for enlist that I've seen before include "Engage" , "Draft" , "Obtain help of" , "Enrol in the services" , "Join the armed forces" .)