Children watch this sad boy (4,5)
I believe the answer is:
blue peter
'children watch this' is the definition.
(children's TV programme)
'sad boy' is the wordplay.
'sad' becomes 'blue' (similar in meaning).
'boy' becomes 'peter' ().
'blue'+'peter'='BLUE PETER'
(Other definitions for blue peter that I've seen before include "Flag to show a vessel is ready to sail" , "Flag showing ship is about to leave" , "Flag raised before sailing" , "Flag on ship about to leave port" , "Flag flown when about to sail - TV programme" .)