Pervasive crackers met in person (11)
I believe the answer is:
omnipresent
'pervasive' is the definition.
(thesaurus)
'crackers met in person' is the wordplay.
'crackers' indicates an anagram (informal term for mad).
'met'+'in'+'person'='metinperson'
'metinperson' with letters rearranged gives 'OMNIPRESENT'.
(Other definitions for omnipresent that I've seen before include "Found everywhere" , "Constantly encountered" , "Ubiquitous" , "(Of God) everywhere simultaneously" , "Everywhere at the same time" .)