Up in arms, stupid nit drinking like granny (9)
I believe the answer is:
indignant
'up in arms' is the definition.
(I've seen this before)
'stupid nit drinking like granny' is the wordplay.
'stupid' indicates an anagram (in a stupid order).
'drinking' means one lot of letters goes inside another (inserted letters are consumed or drunk).
'like' becomes 'dig' (to dig can mean to appreciate or 'get').
'granny' becomes 'nan'.
'nit' is an anagram of 'int'.
'dig'+'nan'='dignan'
'int' placed around 'dignan' is 'INDIGNANT'.
(Other definitions for indignant that I've seen before include "somewhat angry" , "Angered by unjust treatment" , "Felling anger over something wrong or unjust" , "Annoyed at unfair treatment" , "Affronted by perceived injustice" .)