Appeared to join with editor outside (7)
I believe the answer is:
emerged
'appeared' is the definition.
(emerging is a kind of appearing)
'join with editor outside' is the wordplay.
'join with' becomes 'merge' (to merge is to join with something else).
'editor' becomes 'ed'.
'outside' means one lot of letters goes inside another.
'merge' put within 'ed' is 'EMERGED'.
'to' is the link.
(Other definitions for emerged that I've seen before include "Slowly came out" , "Came out, became known" , "Came forth" , "Came into view or became apparent" , "Emanated" .)