Call it up, Charlie, in this alphabet (8)
I believe the answer is:
phonetic
'this alphabet' is the definition.
(phonetic alphabet is a kind of alphabet)
'call it up charlie' is the wordplay.
'call' becomes 'phone' ('phone call' can be a synonym of 'call'**).
'up' is a reversal indicator (in down clue: letters go upwards).
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'it' written backwards gives 'ti'.
'phone'+'ti'+'c'='PHONETIC'
'in' is the link.
(Other definitions for phonetic that I've seen before include "Relating to speech sounds" , "representing vocal sounds" , "Relating to the sounds of spoken language" , "Relating to the sound of words" , "(Spelling) as pronounced" .)