Sailor is behind thorns design for Polaris (5,4)
I believe the answer is:
north star
'for polaris' is the definition.
(synonyms)
'sailor is behind thorns design' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'is behind' means one lot of letters go next to another.
'design' is an anagram indicator.
'thorns' is an anagram of 'norths'.
'tar' put after 'norths' is 'NORTH STAR'.
(Other definitions for north star that I've seen before include "Navigational aid" , "Polaris" , "nickname of US State" , "navigational guide" , "feature of Ursa Minor" .)