The most common and most likely answer to this crossword clue is the 6 letter word PIANOS
P
I
A
N
O
S
Buddy explains!
The straight part
We know that "They could be grand" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they could be grand = pianos.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 PIANOS mean?
PIANOS noun- A percussive keyboard musical instrument, usually ranging over seven octaves, with white and black colored keys, played by pressing these keys, causing hammers to strike strings