Possible Clues:

• Substitute
• Temp
• Take the place of figurehead not well at home
• Complete, as a crossword
• Step into someone's boots almost providing help for restoring The Crown?
• Offend in a bad way at home under study?
• Act as a substitute
• Deputise - complete form
• Substitute not quite satisfying hunger
• In what's on the fish I will insert what's missing