The most common and most likely answer to this crossword clue is the 8 letter word FLATTEST
I've seen this crossword clue in The Guardian
F
L
A
T
T
E
S
T
Buddy explains!
The straight part
We know that "Most insipid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most insipid = flattest.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 FLATTEST mean?
FLATTEST adjective- Having no variations in height.
- (voice) Without variations in pitch.
- Having small or invisible breasts and/or buttocks.
- (note) Lowered by one semitone.
- Of a note or voice, lower in pitch than it should be.
- (of a tire or other inflated object) Deflated, especially because of a puncture.
- Uninteresting.
- (of a carbonated drink) With all or most of its carbon dioxide having come out of solution so that the drink no longer fizzes or contains any bubbles.
- Lacking acidity without being sweet.
- (of a battery) Unable to emit power; dead.
- (of a throw) Without spin; spinless.
- Lacking liveliness or action; depressed; dull and boring.
- Absolute; downright; peremptory.
- (of a consonant) sonant; vocal, as distinguished from a sharp (non-sonant) consonant
- (grammar) Not having an inflectional ending or sign, such as a noun used as an adjective, or an adjective as an adverb, without the addition of a formative suffix; or an infinitive without the sign "to".
- (of a golf club) Having a head at a very obtuse angle to the shaft.
- (of certain fruits) Flattening at the ends.
- (authorship, especially of a character) Lacking in depth, substance, or believability; underdeveloped; one-dimensional.
- Fixed; unvarying.