A choice of letters is the usual thing (4)
I believe the answer is:
norm
I believe this is a quadruple definition!!
'a choice' is the first definition.
(I have seen 'Standard choice' mean 'norm' so perhaps 'choice' could also mean 'norm')
'of' acts as a link.
'letters' is the second definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'is the usual' is the third definition.
(I've seen this before)
'thing' is the fourth definition.
(I have seen 'Usual thing ' mean 'norm' so perhaps 'thing' could also mean 'norm')
Can you help me to learn more?
(Other definitions for norm that I've seen before include "Standard regarded as typical" , "expected thing" , "Drawing stick" , "Required or acceptable standard" , "(The) accepted way" .)