The most common and most likely answer to this crossword clue is the 3 plus 2 plus 3 letter word BIT BY BIT
I've seen this crossword clue in The Daily Telegraph
B
I
T
B
Y
B
I
T
More clues leading to the result BIT BY BIT
Buddy explains!
The straight part
We know that "Two adjacent pieces /a gradual sequence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case two adjacent pieces /a gradual sequence = bit by bit.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.