Gallant chap in pain (7)
I believe the answer is:
paladin
'gallant' is the definition.
(thesaurus)
'chap in pain' is the wordplay.
'chap' becomes 'lad'.
'in' indicates putting letters inside.
'lad' put within 'pain' is 'PALADIN'.
(Other definitions for paladin that I've seen before include "One of the twelve peers at Charlemagne's court - heroic champion" , "paragon" , "Medieval champion" , "Heroic knight - aid plan (anag)" , "Knight errant; a peer of Charlemagne" .)