Strong: alternatively, on reflection, broken (6)
I believe the answer is:
robust
'strong' is the definition.
('robust' can be similar in meaning to 'strong')
'alternatively on reflection broken' is the wordplay.
'alternatively' becomes 'or'.
'on reflection' says the letters should be written in reverse.
'broken' becomes 'bust' (I've seen this before).
'or' back-to-front is 'ro'.
'ro'+'bust'='ROBUST'
(Other definitions for robust that I've seen before include "Vigorous and hearty" , "reliable?" , "of sturdy construction" , "No-nonsense" , "firm!" .)