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' is an anagram indicator.
'broken' becomes 'bust' (bust can mean broken).
'or' is an anagram of 'ro'.
'ro'+'bust'='ROBUST'
(Other definitions for robust that I've seen before include "of sturdy construction" , "Tough!" , "Physically strong" , "'Hearty, healthy (6)'" , "firm!" .)