I'm not sure penetrating this setter is simple (4)
I believe the answer is:
mere
'is simple' is the definition.
(synonyms)
'i'm not sure penetrating this setter' is the wordplay.
'i'm not sure' becomes 'er' (expression of hesitation).
'penetrating' indicates putting letters inside.
'this setter' becomes 'me' (person setting the crossword).
'er' placed into 'me' is 'MERE'.
(Other definitions for mere that I've seen before include "Lake, pond (poetic)" , "Paltry" , "Without additions or modifications" , "Lake - French mother" , "sea" .)