Challenge and fail to grasp leader in Sunday Express (7)
I believe the answer is:
dispute
'challenge' is the definition.
(I know that challenge can be written as dispute)
'fail to grasp leader in sunday express' is the wordplay.
'fail' becomes 'die' (synonyms).
'to grasp' is an insertion indicator.
'leader in' indicates taking the first letters.
'express' becomes 'put' (I've seen this in another clue).
The initial letter of 'sunday' is 's'.
's'+'put'='sput'
'die' placed around 'sput' is 'DISPUTE'.
'and' is the link.
(Other definitions for dispute that I've seen before include "Altercate" , "Row" , "Argue about (something)" , "Quarrel over rights" , "Question or deny accuracy or validity" .)