The clue phrases followed by (X.Y) each had a two-word answer that consisted of words of length X that are identical except for the letter in position Y. (hinted at in the puzzle’s title, “Tit for Tat”). These were

Drivers destabilise? (6.4)
CARMEN CAREEN

Help a top-roper with your gut? (5.4)
BELLY BELAY

Weak point of a lever? (5.4)
CRANK CRACK

Reorganises puffy desserts? (8.2)
SHUFFLES SOUFFLES

What you need to get the flattest dough? (5.1)
LEAST YEAST

Numerically, the body of water coming after the one at Edinburgh? (5.3)
FIFTH FIRTH

2-for-1 on name-brand soap? (4.2)
DIAL DEAL

Nirvana feud? (6.4)
GRUNGE GRUDGE

The differing letters spell out MELANCHOLYFRIEND, and we are left with “(6.1)” to suggest that this is a final clue. We expected SOMBRE HOMBRE to be the solution folks would find, but somewhat surprisingly a second solution was equally popular and legitimate enough that we accepted it as well… which was MELLOW FELLOW. Incredibly, the initials of the clue phrase “melancholy friend” matched the distinct letters in this unintended solution.

The following puzzlers managed to find at least one of the two answer phrases above.