Subject of a Canaletto picture (5)
I believe the answer is:
topic
'subject' is the definition.
('topic' can be a synonym of 'subject')
'of a canaletto picture' is the wordplay.
'of' says the answer is hidden in the clue.
'TOPIC' is hidden within 'canaletto picture'.
(Other definitions for topic that I've seen before include "Subject (eg, of text)" , "might this be hot?" , "Theme of discourse" , "Subject matter, theme" , "Issue (for discussion)" .)