Mock intermittently flawless English (5)
I believe the answer is:
false
'mock' is the definition.
(thesaurus)
'intermittently flawless english' is the wordplay.
'intermittently' indicates alternate letters.
'english' becomes 'e' (abbreviation).
The alternating letters of 'flawless' are 'fals'.
'fals'+'e'='FALSE'
(Other definitions for false that I've seen before include "Artificial" , "Not in accordance with fact" , "Fake; untrue" , "Bogus" , "Invalid; untrue" .)