Bit of speech conveyed by telephone message (7)
I believe the answer is:
phoneme
'bit of speech' is the definition.
(a phoneme is a speech sound)
'conveyed by telephone message' is the wordplay.
'conveyed by' indicates the answer is hidden within the clue.
'PHONEME' is hidden within 'telephone message'.
(Other definitions for phoneme that I've seen before include "Unit of spoken sound" , "minimal piece of speech" , "distinctive sound" , "Small bit of speech" .)