Look upon as a simpleton (4)
I believe the answer is:
loon
'a simpleton' is the definition.
(I've seen this in another clue)
'look upon' is the wordplay.
'look' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'upon' becomes 'on' (I've seen this in another clue).
'lo'+'on'='LOON'
'as' is the link.
(Other definitions for loon that I've seen before include "Simple-minded person" , "Noddy" , "N American diving bird" , "US bird is also a silly person?" , "Diver; silly person" .)