Apprehensive over sun treatment (7)
I believe the answer is:
nervous
'apprehensive' is the definition.
(synonyms)
'over sun treatment' is the wordplay.
'treatment' indicates anagramming the letters (letters treated or processed differently).
'over'+'sun'='oversun'
'oversun' is an anagram of 'NERVOUS'.
(Other definitions for nervous that I've seen before include "Fidgety" , "Highly strung" , "Feeling or showing anxiety" , "Easily alarmed" , "Jittery" .)