Quiet, more mature, say, accepting you assume responsibility (8)
I believe the answer is:
shoulder
'assume responsibility' is the definition.
'shoulder' can be an answer for 'assume' (shoulder can mean to assume or take on). I am unsure of the 'responsibility' bit.
'quiet more mature say accepting' is the wordplay.
'quiet' becomes 'sh' ('sh' is used to tell someone to be quiet).
'more mature' becomes 'older' ('old' is associated in meaning with 'mature').
'say' becomes 'u' (I can't justify this - if you can you should give a lot more credence to this answer).
'accepting' means one lot of letters goes inside another.
'older' going around 'u' is 'oulder'.
'sh'+'oulder'='SHOULDER'
'you' is the link.
Can you help me to learn more?
(Other definitions for shoulder that I've seen before include "take responsibility?" , "Old usher, oddly, is part of the body" , "Top of the arm" , "Cut of meat" , "Part of the body (that bears burdens?)" .)