Ideal chap wears some jewellery (7)
I believe the answer is:
optimal
'ideal' is the definition.
(I've seen this before)
'chap wears some jewellery' is the wordplay.
'chap' becomes 'tim' (short for Timothy).
'wears' is an insertion indicator.
'some jewellery' becomes 'opal' (I've seen this before).
'tim' going within 'opal' is 'OPTIMAL'.
(Other definitions for optimal that I've seen before include "Best possible" , "Most desirable" , "top, surprisingly" , "best results" , "Best or most favourable" .)