Strong, or possibly broken (6)
I believe the answer is:
robust
'strong' is the definition.
(similar in meaning)
'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 "Down-to-earth" , "Well-built" , "Rugged and sturdy" , "Full and rich (wine)" , "Sturdy, resilient" .)