Less sensitive about English leader (6,3)
I believe the answer is:
number one
'leader' is the definition.
(I've seen this before)
'less sensitive about english' is the wordplay.
'less sensitive' becomes 'number' (numb means insensitive or unfeeling).
'about' becomes 'on' (as in a book 'on' a certain topic).
'english' becomes 'E' (abbreviation).
'number'+'on'+'e'='NUMBER ONE'
(Other definitions for number one that I've seen before include "the most important person" , "Wee" , "it concerns egoists" , "best record" , "Naval first lieutenant; foremost person/thing" .)