Simpleton — one with a straw in his mouth? (6)
I believe the answer is:
sucker
'straw in his mouth?' is the definition.
'sucker' can be an answer for 'straw' (a straw is used to suck a drink). I am not certain of the 'in his mouth?' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sucker that I've seen before include "Slangily, one who is easily deceived" , "Underground shoot from shrub, or slang victim" , "Slangy victim or plant shoot" , "Fool" , "Gullible person; plant shoot" .)