Experienced editor supporting main issue (8)
I believe the answer is:
seasoned
'experienced' is the definition.
(similar in meaning)
'editor supporting main issue' is the wordplay.
'editor' becomes 'ed' (abbreviation).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'main' becomes 'sea' (the main is an old name for the sea).
'issue' becomes 'son' (issue can mean someone's offspring).
'sea'+'son'='season'
'ed' after 'season' is 'SEASONED'.
(Other definitions for seasoned that I've seen before include "Gave experience to" , "Experienced with condiments added?" , "Flavoured or hardened" , "Tempered" , "Mature" .)