Related Clues:

• AU? That's it
• On board, yes, at the double
• Yes, this might make 21 across quite sound
• Yes, it's on board for one to monkey with
• Be agreeable at the double on board
• Sounds like what you need two of for 15 down? Yes
• When on board, the primate has to obey like this
• On board it is agreeable to have a primate
• By the sound of it, one might look so favourably thus
• Yes, on board it's coming at the double for the primate
• The primate is agreeable on board
• Acknowledgement to the captain
• Sailor's "yes"
• Agreement from one's mate?
• Sailor's "sure"