Sturdy old boy in reddish-brown (6)
I believe the answer is:
robust
'sturdy' is the definition.
(similar in meaning)
'old boy in reddish-brown' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'in' indicates putting letters inside.
'reddish-brown' becomes 'rust' (I've seen this in another clue).
'ob' inserted into 'rust' is 'ROBUST'.
(Other definitions for robust that I've seen before include "Having a strong constitution" , "Well-built" , "Study and durable" , "Down-to-earth" , "Physically strong" .)