Justify old patent (7)
I believe the answer is:
explain
'justify' is the definition.
(explaining is a kind of justifying)
'old patent' is the wordplay.
'old' becomes 'ex'.
'patent' becomes 'plain' ('plain' can be a synonym of 'patent').
'ex'+'plain'='EXPLAIN'
(Other definitions for explain that I've seen before include "Defend" , "Describe" , "offer clarification" , "Show the meaning of, make clear" , "Show the whys and wherefores" .)