Channel 50: see Doctor Who outside (6)
I believe the answer is:
hollow
'channel' is the definition.
(channel can mean a hollow or trench)
'50 see doctor who outside' is the wordplay.
'50' becomes 'l' (Roman numeral).
'see' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'doctor' indicates an anagram.
'outside' indicates putting letters inside.
'who' with letters rearranged gives 'how'.
'l'+'lo'='llo'
'llo' inserted inside 'how' is 'HOLLOW'.
(Other definitions for hollow that I've seen before include "Having a cavity inside" , "Insincere; not solid" , "Not solid, having hole in the middle" , "Dent; echoey" , "Empty, not solid" .)