Champion of a chap in pain (7)
I believe the answer is:
paladin
'champion' is the definition.
(synonyms)
'chap in pain' is the wordplay.
'chap' becomes 'lad'.
'in' means one lot of letters goes inside another.
'lad' inserted inside 'pain' is 'PALADIN'.
'of a' acts as a link.
(Other definitions for paladin that I've seen before include "paragon" , "Chivalrous knight" , "Medieval champion" , "Heroic knight - aid plan (anag)" , "Knight errant; a peer of Charlemagne" .)