One wandering sailor in van with dog barking (8)
I believe the answer is:
vagabond
'one wandering' is the definition.
(I've seen this before)
'sailor in van with dog barking' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'in' is an insertion indicator.
'with' means one lot of letters go next to another.
'barking' is an anagram indicator (UK slang for mad).
'van'+'dog'='vandog'
'vandog' anagrammed gives 'vagond'.
'ab' put into 'vagond' is 'VAGABOND'.
(Other definitions for vagabond that I've seen before include "An idle or dishonest wanderer" , "Homeless wanderer" , "An idle or dishonest vagrant" , "One having no settled home" , "One wandering" .)