Formal celeb first to claim healthy skin (7)
I believe the answer is:
starchy
'formal' is the definition.
(similar in meaning)
'celeb first to claim healthy skin' is the wordplay.
'celeb' becomes 'star' (I've seen this in another clue).
'first to' suggests taking the first letters.
'skin' means to remove the middle letters (only the skin or outside of the word is used).
'healthy' with its middle taken out is 'hy'.
The first letter of 'claim' is 'c'.
'star'+'c'+'hy'='STARCHY'
(Other definitions for starchy that I've seen before include "Stiff, formal" , "Stiff and formalistic" , "Unbending" , "Stiff, like potatoes" , "Conventional" .)