Strongly built den about right (6)
I believe the answer is:
sturdy
'strongly built' is the definition.
(I've seen this before)
'den about right' is the wordplay.
'den' becomes 'study' (I've seen this before).
'about' means one lot of letters goes inside another.
'right' becomes 'r' (common abbreviation).
'study' enclosing 'r' is 'STURDY'.
(Other definitions for sturdy that I've seen before include "Powerful" , "Stout, brawny" , "Hardy, stalwart" , "Firm" , "Strong and sturdily built" .)