Fiddling cars for a friend is dishonest (8)

'dishonest' is the definition.
('rascally' can be similar in meaning to 'dishonest')

'fiddling cars for a friend' is the wordplay.
'fiddling' indicates an anagram.
'for' is a charade indicator (letters next to each other).
'a friend' becomes 'ally' ('ally' can be a synonym of 'friend').
'cars' with letters rearranged gives 'rasc'.
'rasc'+'ally'='RASCALLY'

'is' acts as a link.