Act One is in a different vein; languid and lackadaisical (8)
I believe the answer is:
inactive
'lackadaisical' is the definition.
(thesaurus)
'act one is in a different vein' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is in' means one lot of letters goes inside another.
'a different' is an anagram indicator (letters placed in a different order).
'act'+'i'='acti'
'vein' is an anagram of 'inve'.
'acti' going inside 'inve' is 'INACTIVE'.
'languid and' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for inactive that I've seen before include "Idle, showing no movement" , "Passive, doing nothing" , "Still" , "Not operative" , "Lifeless" .)