It's damp across hill before hint of sun for travellers (9)

'for travellers' is the definition.
(people travelling in cars)

'it's damp across hill before hint of sun' is the wordplay.
'it's damp' becomes 'moist' (synonyms).
'across' is an insertion indicator.
'hill' becomes 'tor' (tor is a kind of hill).
'before' is a charade indicator (letters next to each other).
'hint of' says to take the initial letters (the smallest possible amount of the word).
The first letter of 'sun' is 's'.
'moist' placed around 'tor' is 'motorist'.
'motorist'+'s'='MOTORISTS'