It's a canal, possibly devilish (9)
I believe the answer is:
satanical
'devilish' is the definition.
(I've seen this in another clue)
'it's a canal possibly' is the wordplay.
'possibly' is an anagram indicator.
'its'+'a'+'canal'='itsacanal'
'itsacanal' is an anagram of 'SATANICAL'.
(Other definitions for satanical that I've seen before include "Hellish" , "Extremely wicked" , "Devilish" , "Devilish,evil" .)