Possible Clues:

• It needs this to make things as strong as the end of this
• One may make it because it's in 30 across
• The strength you need to make the place strong at last
• It needs strength to make this strong at last
• Strength is needed to make it strong at last
• A bid or endeavour
• You have to be strong to make it strong at last
• Strenuous endeavour
• One might try to make it, because it's in the newt
• It takes strength to make it so strong at last