Lose firmness

SAG

Think of how an old mattress or a hammock might look when it can no longer hold its shape under weight.

Lose firmness" is a straightforward definition clue pointing to "SAG," which means to droop or become less taut over time. The word fits naturally—think of fabric sagging on a clothesline or a mattress losing its support. There’s no hidden wordplay here; it’s a direct match between the clue’s meaning and the answer.