Watched you once being introduced to editor (4)
I believe the answer is:
eyed
'watched' is the definition.
(I've seen this before)
'you once being introduced to editor' is the wordplay.
'you once' becomes 'ye' (historical version of 'you').
'being introduced to' means one lot of letters goes inside another.
'editor' becomes 'ed' (abbreviation).
'ye' put within 'ed' is 'EYED'.
(Other definitions for eyed that I've seen before include "Viewed" , "Looked at, warily perhaps" , "examined" , "Watched warily" , "Watched, looked at" .)