The most common and most likely answer to this crossword clue is the 10 letter word LEFT-FIELD
I've seen this crossword clue in The Guardian
L
E
F
T
-
F
I
E
L
D
More clues leading to the result LEFT-FIELD
Buddy explains!
The straight part
We know that "that’s not conventional" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s not conventional = left-field.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.