The most common and most likely answer to this crossword clue is the 12 letter word AMATEURISHLY
A
M
A
T
E
U
R
I
S
H
L
Y
More clues leading to the result AMATEURISHLY
Buddy explains!
The straight part
We know that "in an unprofessional manner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in an unprofessional manner = amateurishly.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.