Illuminated, chap became graceful (5)
I believe the answer is:
lithe
'graceful' is the definition.
(similar in meaning)
'illuminated chap' is the wordplay.
'illuminated' becomes 'lit' (synonyms).
'chap' becomes 'he'.
'lit'+'he'='LITHE'
'became' is the link.
(Other definitions for lithe that I've seen before include "Moving and bending with ease" , "Slim and supple" , "grace" , "Slender, supple" , "Physically flexible or supple" .)