Slim women -- not in twelves, possibly! (6)
I believe the answer is:
svelte
'slim' is the definition.
(I know that slim can be written as svelte)
'women not in twelves possibly' is the wordplay.
'women' becomes 'w'.
'not in' suggests deleting specific letters.
'possibly' is an anagram indicator.
'twelves' with 'w' removed is 'telves'.
'telves' anagrammed gives 'SVELTE'.
(Other definitions for svelte that I've seen before include "Of delicate or slender build" , "Slender - refined" , "Sylphlike" , "Slim and elegant" , "Slim and attractive" .)