Fools do, and it is silly (6)
I believe the answer is:
idiots
'fools' is the definition.
(I've seen this before)
'do and it is silly' is the wordplay.
'and' means one lot of letters go next to another.
'silly' is an anagram indicator.
'do'+'it'+'is'='doitis'
'doitis' is an anagram of 'IDIOTS'.
(Other definitions for idiots that I've seen before include "Abusive word for unintelligent or irrational people" , "Fools, air-heads" , "clowns?" , "imbeciles" , "chumps" .)