I believe that the key to the riddle, and potentially the solution, is
Double Letter
Meaning that
All of the solution words likely contain a double letter, or at least two of the same letter.
Work so far:
All of forever. (8)
Utter (eternity?) --> T
It's big enough to grab! (8)
(immense/massive) ????? --> M/S
Look at the front face. (7)
See ????? --> E
Merchant awaiting feathers. (6)
(Quill?) Seller (although this does not fit "awaiting" --> L
Mounted when off the boat. (6)
Saddled ????? --> D
Museum air vent. (6)
Gallery (Ventilator?) --> L
Othello editor. (7)
Moor (Reviser?) --> O
Smart powers of two. (6)
Keen Evens --> E
So far, that gives us the letters
T M/S E L D L O E
If we assume the solution to be "DOUBLE LETTER", then we already have "D---LE LET---" with some letters potentially wrong. It seems possible that some of the clues might provide two different letters, while some might only provide one. I'm sure someone else can do a better job of finishing/revising my current guesses. Additionally, I'm not sure what the numbers beside each clue represent. Perhaps those are meant to be the number of repeated letters per clue solution?