The most common and most likely answer to this crossword clue is the 5 plus 2 plus 5 letter word PIECE BY PIECE
I've seen this crossword clue in Financial Times
P
I
E
C
E
B
Y
P
I
E
C
E
Buddy explains!
The straight part
We know that "Gradually/how to score at Scrabble" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gradually/how to score at scrabble = piece by piece.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.