Conceited editor to support significant source (9)
I believe the answer is:
bigheaded
'conceited' is the definition.
(I've seen this before)
'editor to support significant source' is the wordplay.
'editor' becomes 'ed'.
'to support' means one lot of letters go next to another (in a down clue, some letters hold others up).
'significant' becomes 'big' ('big' can be similar in meaning to 'significant').
'source' becomes 'head' (head is a kind of source**).
'big'+'head'='bighead'
'ed' put after 'bighead' is 'BIGHEADED'.
(Another definition for bigheaded that I've seen is " Conceited with large cranium".)