Channel, small, across the pond (5)
I believe the answer is:
sinus
'channel' is the definition.
(sinus is a kind of channel)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for sinus that I've seen before include "Cavity in skull connecting with nostrils" , "Cavity in body (connecting to nose)" , "Hole in the head" , "chamber" , "Cavity in the facial bones" .)