Linguistic device sound on reflection: large one included (7)
I believe the answer is:
elision
'linguistic device' is the definition.
(I know that elision is a linguistic device)
'sound on reflection large one included' is the wordplay.
'sound' becomes 'noise' (a noise is a sound).
'on reflection' says the letters should be written in reverse.
'large' becomes 'L' (eg in clothes sizes).
'one' becomes 'i' (Roman numeral).
'included' is an insertion indicator.
'noise' in reverse letter order is 'esion'.
'l'+'i'='li'
'esion' enclosing 'li' is 'ELISION'.
(Other definitions for elision that I've seen before include "Omission of syllables in speech" , "Dropping of vowel" , "Omission (of a syllable)" , "Omission of a sound in speech" , "Omission (from a book, etc)" .)