Most excited by a gap in botany (5)

I believe the answer is:

stoma

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'gap in botany' is the definition.
'stoma' can be an answer for 'gap' (stoma is a kind of gap). I am not sure about the 'in botany' bit.

'most excited by a' is the wordplay.
'excited' indicates an anagram.
'by' says to put letters next to each other.
'most' anagrammed gives 'stom'.
'stom'+'a'='STOMA'

Can you help me to learn more?

(Other definitions for stoma that I've seen before include "Pore in plant" , "surgical opening" .)

I've seen this clue in The Daily Express.

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