Justify being old and unattractive (7)
I believe the answer is:
explain
'justify' is the definition.
(I know that explain is a kind of justify)
'old and unattractive' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'and' is a charade indicator (letters next to each other).
'unattractive' becomes 'plain' (similar in meaning).
'ex'+'plain'='EXPLAIN'
'being' is the link.
(Other definitions for explain that I've seen before include "Pin axle (anag.)" , "Shed light" , "Show the whys and wherefores" , "Give reasons for" , "Make intelligible and account for" .)