The most common and most likely answer to this crossword clue is the 8 plus 4 letter word ABNORMAL LOAD
A
B
N
O
R
M
A
L
L
O
A
D
Buddy explains!
The straight part
We know that "it might move very slowly!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it might move very slowly! = abnormal load.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.