Join up and listen out (6)
I believe the answer is:
enlist
'join up' is the definition.
(I've seen this in another clue)
'listen out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'listen' with letters rearranged gives 'ENLIST'.
'and' acts as a link.
(Other definitions for enlist that I've seen before include "Join up, sign on" , "Engage" , "Join the military" , "Draft" , "Register" .)