I do it foolishly, being a fool (5)
I believe the answer is:
idiot
'being a fool' is the definition.
(I've seen this before)
'i do it foolishly' is the wordplay.
'foolishly' is an anagram indicator.
'do'+'it'='doit'
'doit' anagrammed gives 'diot'.
'i'+'diot'='IDIOT'
(Other definitions for idiot that I've seen before include "Foolish person" , "Charlie" , "Stupid person" , "one needing head examined?" , "Dostoevsky hero" .)