The most common and most likely answer to this crossword clue is the 2 plus 8 letter word CO OPERATOR
I've seen this crossword clue in Financial Times
C
O
O
P
E
R
A
T
O
R
Buddy explains!
The straight part
We know that "One working closely with another" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one working closely with another = co operator.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.