The most common and most likely answer to this crossword clue is the 7 letter word OPERAND
O
P
E
R
A
N
D
More clues leading to the result OPERAND
Buddy explains!
The straight part
We know that "3 + 6 = 9. One of them in the left hand side" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 3 + 6 = 9. one of them in the left hand side = operand.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 OPERAND mean?
OPERAND noun- A quantity to which an operator is applied (in 3 - x, the operands of the subtraction operator are 3 and x).