Silly mode of expression cut short by nervous reaction (7)
I believe the answer is:
idiotic
'silly' is the definition.
(I've seen this before)
'mode of expression cut short by nervous reaction' is the wordplay.
'mode of expression' becomes 'idiom' (I've seen this before).
'cut short' means to remove the last letter.
'by' says to put letters next to each other.
'nervous reaction' becomes 'tic' (I've seen this in other clues).
'idiom' with its final letter removed is 'idio'.
'idio'+'tic'='IDIOTIC'
(Other definitions for idiotic that I've seen before include "Inviting ridicule" , "description of Dostoevsky hero?" , "Senseless" , "Mindless" , "Silly" .)