Fretful about underground fungus (7)

I believe the answer is:

truffle

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'underground fungus' is the definition.
'truffle' can be an answer for 'fungus' (truffle is a kind of fungus). I'm not sure about the 'underground' bit.

'fretful about' is the wordplay.
'about' indicates anagramming the letters.
'fretful' is an anagram of 'TRUFFLE'.

Can you help me to learn more?

(Other definitions for truffle that I've seen before include "2 (anag)" , "fungal growth" , "Underground delicacy" , "Edible fungus -- chocolate sweet" , "Valued culinary fungus" .)

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