Poor on crime, do change English leader (8)
I believe the answer is:
mediocre
'poor' is the definition.
('mediocre' can be a synonym of 'poor')
'crime do change english leader' is the wordplay.
'change' indicates an anagram.
'leader' says to take the initial letters.
The initial letter of 'english' is 'e'.
'crime'+'do'='crimedo'
'crimedo' anagrammed gives 'mediocr'.
'mediocr'+'e'='MEDIOCRE'
'on' acts as a link.
(Other definitions for mediocre that I've seen before include "Of only average quality" , "poor to middling" , "Commonplace, not distinguished" , "Middling or average" , "Rather inferior" .)