I do it when playing the fool (5)
I believe the answer is:
idiot
'the fool' is the definition.
(synonyms)
'i do it when playing' is the wordplay.
'when playing' is an anagram indicator.
'do'+'it'='doit'
'doit' with letters rearranged gives 'diot'.
'i'+'diot'='IDIOT'
(Other definitions for idiot that I've seen before include "Twit" , "Fool, silly person" , "Ignoramus's" , "Imbecile" , "Dunce" .)