The most common and most likely answer to this crossword clue is the 13 letter word PROPORTIONATE
I've seen this crossword clue in The Daily Telegraph
P
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Buddy explains!

The straight part
We know that "In scale" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in scale = proportionate.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.
What does PROPORTIONATE mean?
PROPORTIONATE verb- To make proportionate.
- In proportion; proportional; commensurable.
- Harmonious and symmetrical.