The most common and most likely answer to this crossword clue is the 17 letter word SECOND-IN-COMMAND
I've seen this crossword clue in The Times
S
E
C
O
N
D
-
I
N
-
C
O
M
M
A
N
D
More clues leading to the result SECOND-IN-COMMAND
Buddy explains!

The straight part
We know that "deputy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deputy = second-in-command.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 SECOND-IN-COMMAND mean?
SECOND-IN-COMMAND noun- Somebody ranking next below a commander.
- Someone ranking next below the person in charge.