Again rouses prominent scientists to host a late party in small space (9)

'again rouses' is the definition.
(wakes up again)

'prominent scientists to host a late party in small space' is the wordplay.
'prominent scientists' becomes 'RS' (Royal Society).
'to host' is an insertion indicator.
'late party' becomes 'wake' (event after a death - late can mean dead).
'in' means one lot of letters goes inside another.
'small space' becomes 'en' (in printing, a space as wide as a letter 'n').
'a'+'wake'='awake'
'awake' going inside 'en' is 'eawaken'.
'rs' enclosing 'eawaken' is 'REAWAKENS'.