Poor on crime, do change English leader (8)
I believe the answer is:
Here is my best explanation:
'poor' is the definition.
('mediocre' can be a synonym of 'poor')
'crime do change english leader' is the subsidiary indication.
'change' is an anagram indicator.
'leader' indicates taking the first letters.
The initial letter of 'english' is 'e'.
'crimedo' is an anagram of 'mediocr'.
'on' is the link.
(Other definitions for mediocre that I've seen before include "Pedestrian, second-rate", "Middling or average", "Rather poor", "Somewhat inferior", "Come ride (anag.)", "Moderate to inferior in quality", "Of unexceptional quality", "poor to middling", "Undistinguished" and "Run-of-the-mill".)