The most common and most likely answer to this crossword clue is the 4 plus 3 plus 4 plus 4 letter word HARD AND FAST RULE
H
A
R
D
A
N
D
F
A
S
T
R
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L
E
More clues leading to the result HARD AND FAST RULE
Buddy explains!
The straight part
We know that "Strict guideline" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case strict guideline = hard and fast rule.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.