Thus PM defends his specious argument (7)
I believe the answer is:
sophism
'specious argument' is the definition.
(I've seen this before)
'thus pm defends his' is the wordplay.
'thus' becomes 'so' (synonyms).
'defends' indicates putting letters inside.
'pm' placed around 'his' is 'phism'.
'so'+'phism'='SOPHISM'
(Other definitions for sophism that I've seen before include "faulty reasoning" , "specious argument" , "False argument" .)