The most common and most likely answer to this crossword clue is the 7 letter word SUPREMO
I've seen this crossword clue in The Daily Telegraph
S
U
P
R
E
M
O
More clues leading to the result SUPREMO
- Drink on tick for boss
- Boss has drink, on second
- Drink more extravagantly as commander
- Big cheese and more crackers after drink
- Leader to drink more, getting drunk
- Master wrestling holds in advance
- Big cheese Diana Ross once cut to get slice of Ossau-Iraty
- Retrospective work covering REM and The Boss?
- Boss promoting European involved in wrestling
- What’s happening about Farah’s boss?
Buddy explains!

The straight part
We know that "C-in-C" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case c-in-c = supremo.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 SUPREMO mean?
SUPREMO noun- The most important person in an organization.