Yes-men always so pleasant? (9)
I believe the answer is:
agreeable
'pleasant?' is the definition.
(similar in meaning)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for agreeable that I've seen before include "delightful" , "Willing - nice" , "In harmony; pleasant" , "sympathetic" , "Pleasant, pleasing" .)