The most common and most likely answer to this crossword clue is the 5 plus 6 letter word PIANO PLAYER
I've seen this crossword clue in The Times
P
I
A
N
O
P
L
A
Y
E
R
Buddy explains!
The straight part
We know that "One putting down a grand on a flat? On the contrary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one putting down a grand on a flat? on the contrary = piano player.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.