Vile reason an earl regularly stripped (8)
I believe the answer is:
infernal
'vile' is the definition.
(both can mean evil)
'reason an earl regularly stripped' is the wordplay.
'reason' becomes 'infer' (both can mean to deduce).
'regularly stripped' indicates alternate letters (letters removed at regular intervals).
'an'+'earl'='anearl'
The alternate letters of 'anearl' are 'nal'.
'infer'+'nal'='INFERNAL'
(Other definitions for infernal that I've seen before include "Of hell, or tiresome like a nuisance" , "evil here" , "Extremely annoying" , "Hell-like" , "Feral inn - it's hellish" .)