Weak peers slow to reform (9)
I believe the answer is:
powerless
'weak' is the definition.
('powerless' can be a synonym of 'weak')
'peers slow to reform' is the wordplay.
'to reform' indicates anagramming the letters.
'peers'+'slow'='peersslow'
'peersslow' is an anagram of 'POWERLESS'.
(Other definitions for powerless that I've seen before include "Weak, impotent" , "Without ability" , "Lacking ability" , "hamstrung" .)