The most common and most likely answer to this crossword clue is the 8 letter word ENTIRELY
I've seen this crossword clue in Financial Times
E
N
T
I
R
E
L
Y
More clues leading to the result ENTIRELY
- Hospital department I trust completely
- Thoroughly scrubbing edges of men’s lavatory, one trusts
- All children tire, lying about
- Hospital department I trust without reservation
- New flag adopted by diocese without reservation
- Fully entitled to leave 50% with bank
- See about inert compound in every way
- All-nite dancing? Count on it
- … only ten pound, I count
- Completely new drain installed in cathedral city
Buddy explains!

The straight part
We know that "Thoroughly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thoroughly = entirely.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.
What does ENTIRELY mean?
ENTIRELY adverb- To the full or entire extent.
- To the exclusion of others.