They may be in bed — one's about to get up (6)
I believe the answer is:
irises
'they may be in bed' is the definition.
The definition suggests an adverb but the answer is not.
'one's about to get up' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'about' means one lot of letters goes inside another.
'to get up' becomes 'rise' (synonyms).
'is' going around 'rise' is 'IRISES'.
Can you help me to learn more?
(Other definitions for irises that I've seen before include "Is sire in favour of flowers?" , "Expensive painting" , "Parts of eyes or flowering plants" , "Flowers and parts of eyes" , "Flags" .)