Sad boy seen when a vessel is about to leave (4,5)
I believe the answer is:
blue peter
'seen when a vessel is about to leave' is the definition.
(flag raised by a ship leaving port)
'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 raised before sailing" , "Flag hoisted when a ship is about to sail" , "Flag flown when about to sail - TV programme" , "Signal flag - children's TV programme" , "Flag showing ship is about to leave" .)