Clue: 0 x 1 — so gradually 5 became 12, 22 26, 1 down 3, or 19 21 (3,2,3)
Answer: BIT BY BIT
Explanation: Definition: gradually
A definition in the middle of the clue, with another (close to a) definition at the start – in computer binary, a BIT may be 0 or 1, and BY is 'x'. The rest is a listing of the four six-letter answer triplets, with the intermediate stage omitted.