The most common and most likely answer to this crossword clue is the 7 letter word VOICING
I've seen this crossword clue in The Times
V
O
I
C
I
N
G
Buddy explains!

The straight part
We know that "Expression" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case expression = voicing.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does VOICING mean?
VOICING verb- To give utterance or expression to; to utter; to publish; to announce
- To utter audibly, with tone and not just breath.
- To fit for producing the proper sounds; to regulate the tone of
- To vote; to elect; to appoint
- To clamor; to cry out
- (IRC) To assign the voice flag to a user on IRC, permitting them to send messages to the channel.
- To act as a voice actor to portray a character.
- The final regulation of the pitch and tone of any sound-producing entity, especially of an organ or similar musical instrument.
- A particular arrangement of notes to form a chord.
- The articulatory process in which the vocal cords vibrate.
- A classification of speech sounds that tend to be associated with vocal cord vibration.
- A phonological process that turns a voiceless sound into a voiced one.