Jack is after small vehicle – a Beetle (6)
I believe the answer is:
scarab
'a beetle' is the definition.
(scarab is a kind of beetle)
'jack is after small vehicle' is the wordplay.
'jack' becomes 'ab' (both can mean a sailor).
'is after' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vehicle' becomes 'car' (car is a kind of vehicle).
's'+'car'='scar'
'ab' put after 'scar' is 'SCARAB'.
(Other definitions for scarab that I've seen before include "Ancient Egyptian symbol" , "Sacred beetle of ancient Egypt" , "Dung beetle - ancient Egyptian cut gem" , "Large dung beetle" , "Type of beetle" .)