Pulling leg, eg a fibber cannot be credible (6,6)
I believe the answer is:
beggar belief
'cannot be credible' is the definition.
'pulling leg eg a fibber' is the wordplay.
'pulling' is an anagram indicator.
'leg'+'eg'+'a'+'fibber'='legegafibber'
'legegafibber' anagrammed gives 'BEGGAR BELIEF'.
(Other definitions for beggar belief that I've seen before include "strain credulity" , "Thus mendicant friars" .)