The most common and most likely answer to this crossword clue is the 8 plus 5 letter word CRITICAL ANGLE
I've seen this crossword clue in The Times
C
R
I
T
I
C
A
L
A
N
G
L
E
Buddy explains!
The straight part
We know that "What reviewer usually offers/maybe beyond which reflection less than total" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what reviewer usually offers/maybe beyond which reflection less than total = critical angle.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.