Flag hoisted when a ship is about to sail (4,5)
I believe the answer is:
blue peter
'flag hoisted when a ship is about to sail' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for blue peter that I've seen before include "Flag on ship about to leave port" , "Ship's flag on leaving port" , "Flag flown to warn of a ship's departure" , "Flag flown when about to leave port - TV programme" , "Flag raised before sailing" .)