The most common and most likely answer to this crossword clue is the 7 letter word AIRHEAD
I've seen this crossword clue in Independent
A
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R
H
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A
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More clues leading to the result AIRHEAD
- Silly sort bearing crown
- Dimwit hairy chef made to strip
- Charlie and Henry into very good book
- Strain on the toilet at sea? Fool
- Feather-brain needs help to catch leggy bird
- Idiot in charge of the RAF?
- Fool? One that might be apparent, we hear, with capital
- Help to catch large bird, one with a small brain
- Sky boss an idiot
- Simple-minded soul in front holding one end of tether
Buddy explains!

The straight part
We know that "one with a small brain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one with a small brain = airhead.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 AIRHEAD mean?
AIRHEAD noun- A horizontal channel providing ventilation in a mine.
- An area of hostile territory that has been seized for use as an airbase to ensure the further safe landing of troops and materiel.
- (by extension) A (usually temporary) landing area for aircraft for supplying a non-military operation.
- (originally United States) A foolish, silly, or unintelligent person.