The most common and most likely answer to this crossword clue is the 6 plus 6 letter word THOMAS EDISON
T
H
O
M
A
S
E
D
I
S
O
N
More clues leading to the result THOMAS EDISON
Buddy explains!
The straight part
We know that "His was no modest invention – overlooking Watt and, originally, Schilling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case his was no modest invention – overlooking watt and, originally, schilling = thomas edison.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.