Tom’s upset about Des being merely middling (6)

I believe the answer is:

modest

I'm a little stuck... Click here to teach me more about this clue!

'merely middling' is the definition.
'modest' can be an answer for 'middling' (thesaurus). I'm not sure about the 'merely' bit.

'tom's upset about des' is the wordplay.
'upset' is a reversal indicator.
'about' is an insertion indicator.
'tom' back-to-front is 'mot'.
'mot' placed around 'des' is 'MODEST'.

'being' acts as a link.

Can you help me to learn more?

(Other definitions for modest that I've seen before include "Unostentatious" , "Not boastful" , "Unassuming or unprententious" , "retiring" , "self-effacing" .)

I've seen this clue in the Evening Standard.

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