One heading for Arctic following sea channels (5)
I believe the answer is:
media
'channels' is the definition.
(I have seen 'Channels of communication ' mean 'media' so perhaps 'channels' could also mean 'media')
'one heading for arctic following sea' is the wordplay.
'one' becomes 'i' (Roman numeral).
'heading for' indicates taking the first letters.
'following' means one lot of letters go next to another.
'sea' becomes 'med' (short for Mediterranean).
The first letter of 'arctic' is 'a'.
'i'+'a'='ia'
'ia' put after 'med' is 'MEDIA'.
(Other definitions for media that I've seen before include "the box etc" , "sources of information" , "Television, papers, etc" , "Reporters" , "Aimed (anag.)" .)