Tighten your belt and behind -- adults are going to be slender (6)
I believe the answer is:
svelte
'slender' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for svelte that I've seen before include "Slim and stylish" , "Sylphlike" , "Slim or slender in a graceful way" , "Slender or sleek" , "Gracefully slim" .)