Deny Conservative leader in Daily Mail is wrong (8)
I believe the answer is:
disclaim
'deny' is the definition.
(disclaiming is a kind of denying)
'conservative leader in daily mail is wrong' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'leader in' indicates taking the first letters.
'wrong' indicates anagramming the letters.
The first letter of 'daily' is 'd'.
'c'+'d'+'mail'+'is'='cdmailis'
'cdmailis' is an anagram of 'DISCLAIM'.
(Other definitions for disclaim that I've seen before include "Forswear" , "Deny or relinquish" , "Disown or deny, maybe responsibility" , "Deny, refuse to acknowledge" , "Renounce" .)