Unwell, not succeeding without leader (6)
I believe the answer is:
ailing
'unwell' is the definition.
('ailing' can be a synonym of 'unwell')
'not succeeding without leader' is the wordplay.
'not succeeding' becomes 'failing' ('fail' is the opposite to 'succeed').
'without leader' means to remove the first letter.
'failing' with its initial letter removed is 'AILING'.
(Other definitions for ailing that I've seen before include "Suffering" , "feeling sick" , "Infertile" , "Troubled in mind or body" , "Unwell, in poor condition" .)