Brown's out to upset Sam, the navigator (6)
I believe the answer is:
tasman
'the navigator' is the definition.
(I've seen this before)
'brown's out to upset sam' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'out' is an insertion indicator.
'to upset' indicates anagramming the letters.
'sam' is an anagram of 'asm'.
'tan' going around 'asm' is 'TASMAN'.
(Other definitions for tasman that I've seen before include "Abel --, Dutch navigator" , "Abel ------, Dutch discoverer of New Zealand" , "Sea named after Dutch navigator" , "Dutch antipodean explorer" , "Dutch sailor" .)