Mug admits quietly slating opening acts (5,2)

'acts' is the definition.
The definition and answer can be both to do with social activities as well as being verbs in their -s form.
Maybe they are linked in a way I don't understand?

'mug admits quietly slating opening' is the wordplay.
'mug' becomes 'stein' (stein is a kind of mug).
'admits' means one lot of letters goes inside another (to admit is to allow in).
'quietly' becomes 'p' (p for piano and quiet).
'opening' says to take the initial letters.
The first letter of 'slating' is 's'.
'p'+'s'='ps'
'stein' going around 'ps' is 'STEPS IN'.

Can you help me to learn more?