Tough or possibly broken (6)
I believe the answer is:
robust
'tough' is the definition.
(robust means tough or strong)
'or possibly broken' is the wordplay.
'possibly' indicates anagramming the letters.
'broken' becomes 'bust' (bust can mean broken).
'or' anagrammed gives 'ro'.
'ro'+'bust'='ROBUST'
(Other definitions for robust that I've seen before include "of sturdy construction" , "Tough!" , "Physically strong" , "'Hearty, healthy (6)'" , "firm!" .)