The most common and most likely answer to this crossword clue is the 6 plus 3 plus 6 letter word PESTLE AND MORTAR
I've seen this crossword clue in The Guardian
P
E
S
T
L
E
A
N
D
M
O
R
T
A
R
Buddy explains!
The straight part
We know that "pair that grind" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pair that grind = pestle and mortar.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.