Boats cross this rebuilt canal -- it's about time (8)
I believe the answer is:
atlantic
'boats cross this' is the definition.
(boats cross the Atlantic Ocean)
'rebuilt canal it's about time' is the wordplay.
'rebuilt' indicates an anagram.
'about' means one lot of letters goes inside another.
'time' becomes 't' (abbreviation).
'canal'+'it'='canalit'
'canalit' with letters rearranged gives 'alantic'.
'alantic' placed around 't' is 'ATLANTIC'.
(Other definitions for atlantic that I've seen before include "Great sea" , "Big sea" , "Latin cat strangely in the sea" , "deep water" , "UK's nearest ocean" .)