Possible Clues:

• Sow for this long spell at the oars
• Long for the oarsman and for that swine
• That's a swine of a long spell at the oars
• Does that mean a lot of work with the oars for 18 down?
• It's for the sow to be having a long spell with the oars
• It's a long way for the oarsman to have to get to the little swine