Well-built old boy in reddish-brown (6)
I believe the answer is:
robust
'well-built' is the definition.
(I've seen this in another clue)
'old boy in reddish-brown' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'in' means one lot of letters goes inside another.
'reddish-brown' becomes 'rust' (I've seen this in another clue).
'ob' placed within 'rust' is 'ROBUST'.
(Other definitions for robust that I've seen before include "firm!" , "Full and rich (wine)" , "Study and durable" , "Athletic" , "Down-to-earth" .)