Pen a part stupidly obvious (8)
I believe the answer is:
apparent
'obvious' is the definition.
(similar in meaning)
'pen a part stupidly' is the wordplay.
'stupidly' indicates anagramming the letters.
'pen'+'a'+'part'='penapart'
'penapart' is an anagram of 'APPARENT'.
(Other definitions for apparent that I've seen before include "Noticeable" , "readily understood" , "See 1" , "Readily perceived" , "That may be seen" .)