Sturdy -- or, on reflection, broken? (6)
I believe the answer is:
robust
'sturdy' is the definition.
('robust' can be similar in meaning to 'sturdy')
'or on reflection broken?' is the wordplay.
'on reflection' is a reversal indicator.
'broken?' becomes 'bust' (I've seen this before).
'or' reversed gives 'ro'.
'ro'+'bust'='ROBUST'
(Other definitions for robust that I've seen before include "Sturdily built" , "'Hearty, healthy (6)'" , "Having a strong constitution" , "No-nonsense" , "Tough!" .)