Bishop, elderly but valiant (4)
I believe the answer is:
bold
'valiant' is the definition.
(I've seen this before)
'bishop elderly' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'elderly' becomes 'old' ('old' can be similar in meaning to 'elderly').
'b'+'old'='BOLD'
'but' is the link.
(Other definitions for bold that I've seen before include "Fearless and daring" , "Typeface with thick heavy lines" , "like Ben Battle" , "No chicken" , "Confident, daring" .)