The most common and most likely answer to this crossword clue is the 7 plus 2 plus 4 letter word DRESSED TO KILL
I've seen this crossword clue in The Sunday Telegraph
D
R
E
S
S
E
D
T
O
K
I
L
L
Buddy explains!
The straight part
We know that "In top gear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in top gear = dressed to kill.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.