He's about to reform Slade without leader (8)
I believe the answer is:
headless
'without leader' is the definition.
('leader' can be 'head' (head is a kind of leader) so maybe 'without leader' can be ''headless')
'he's about to reform slade' is the wordplay.
'about' means one lot of letters goes inside another.
'to reform' indicates an anagram (letters in a new form).
'slade' with letters rearranged gives 'adles'.
'hes' enclosing 'adles' is 'HEADLESS'.
(Other definitions for headless that I've seen before include "Like foolish people" , "Decapitated" , "in want of a leader" .)