Get a shirt undone — one buttoned-up (5-4)

I believe the answer is:

tight-arse

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

'one buttoned-up' is the definition.
I can't tell whether this defines the answer.

'get a shirt undone' is the wordplay.
'undone' is an anagram indicator (the original order is undone).
'get'+'a'+'shirt'='getashirt'
'getashirt' with letters rearranged gives 'TIGHT-ARSE'.

Can you help me to learn more?

Want a hint initially instead of a full solution? Install my app