The most common and most likely answer to this crossword clue is the 10 letter word ESTIMATORS
I've seen this crossword clue in The Times
E
S
T
I
M
A
T
O
R
S
Buddy explains!

The straight part
We know that "We reckon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case we reckon = estimators.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 ESTIMATORS mean?
ESTIMATORS noun- A person who estimates, especially one who estimates costs
- A function of a random sample of a population used to estimate some parameter of the whole population