Arthur’s at sea crossing English Channel (7)
I believe the answer is:
urethra
'channel' is the definition.
(urethra is a kind of channel)
'arthur's at sea crossing english' is the wordplay.
'at sea' indicates anagramming the letters.
'crossing' is an insertion indicator.
'english' becomes 'e' (abbreviation).
'arthur' anagrammed gives 'urthra'.
'urthra' enclosing 'e' is 'URETHRA'.
(Other definitions for urethra that I've seen before include "part of body" , "Canal from the bladder" , "wee channel" , "course of discharge" .)