Simple routine upset about including new start (13)
I believe the answer is:
unpretentious
'simple' is the definition.
('unpretentious' can be similar in meaning to 'simple')
'routine upset about including new start' is the wordplay.
'about' indicates an anagram.
'including' is an insertion indicator.
'start' says to take the initial letters.
The initial letter of 'new' is 'n'.
'routine'+'upset'='routineupset'
'routineupset' with letters rearranged gives 'unpretetious'.
'unpretetious' placed around 'n' is 'UNPRETENTIOUS'.
(Other definitions for unpretentious that I've seen before include "airless?" , "Without airs and graces" .)