In casual manner noble at outset only accepting tea, left walnuts regularly? (12)
I believe the answer is:
nonchalantly
'in casual manner' is the definition.
(nonchalantly matches up with the definition in casual manner)
'noble at outset only accepting tea left walnuts regularly?' is the wordplay.
'at outset' says to take the initial letters.
'accepting' indicates putting letters inside (inserted letters are accepted).
'tea' becomes 'cha' (cha is a type of tea).
'left' becomes 'l' (common abbreviation).
'regularly?' means one should take alternating letters (I've seen 'regularly' mean this (regularly take one letter, leave next etc.)).
The first letter of 'noble' is 'n'.
The alternating letters of 'walnuts' are 'ant'.
'cha'+'l'+'ant'='chalant'
'only' enclosing 'chalant' is 'onchalantly'.
'n'+'onchalantly'='NONCHALANTLY'
(Other definitions for nonchalantly that I've seen before include "in casual fashion" , "In insouciant fashion" , "With composure" , "Casually taking" , "In a calm and relaxed manner" .)