Lasting form of prettiness? (10)
I believe the answer is:
persistent
'lasting' is the definition.
(synonyms)
'form of prettiness?' is the wordplay.
'form of' indicates an anagram (the letters need to be in another 'form').
'prettiness' anagrammed gives 'PERSISTENT'.
(Other definitions for persistent that I've seen before include "Endless" , "Determined" , "Ongoing" , "single-minded" , "Resolute - interminable" .)