The most common and most likely answer to this crossword clue is the 12 letter word DILETTANTISH
I've seen this crossword clue in The Guardian
D
I
L
E
T
T
A
N
T
I
S
H
Buddy explains!
The straight part
We know that "like an amateur" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like an amateur = dilettantish.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.