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 "Moderate to inferior in quality", "Pedestrian, second-rate", "Not very good", "Average (but poor)", "Of unexceptional quality", "Come ride (anag.)", "Nothing special", "Of only average quality", "Very average" and "Fairly bad".)