I can't - it turned out to be vast (7)
I believe the answer is:
titanic
'to be vast' is the definition.
(thesaurus)
'i can't it turned' is the wordplay.
'turned' indicates anagramming the letters.
'i'+'cant'+'it'='icantit'
'icantit' with letters rearranged gives 'TITANIC'.
'out' is the link.
(Other definitions for titanic that I've seen before include "Twenty one [SHIP]" , "Ship sunk by iceberg" , "Iceberg-hitting ship" , "Of great force or power, like the ill-fated ship" , "Exceptionally powerful" .)