Timid at first, mom's opened up since yesterday (5)

I believe the answer is:

mousy

'timid' is the definition.
('mousy' can be similar in meaning to 'timid')

'first mom's opened up since yesterday' is the wordplay.
'first' says to take the initial letters.
The first letters of 'mom opened up since yesterday' is 'MOUSY'.

'at' is the link.

I've seen this clue in The Independent.

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