Sixth sense that's ultimately called "Sighted" (6)
I believe the answer is:
espied
'sighted' is the definition.
(espying is a kind of sighting)
'sixth sense that's ultimately called' is the wordplay.
'sixth sense' becomes 'esp' (I've seen this in other clues).
'that's' becomes 'ie'.
'ultimately' indicates one should take the final letters.
The final letter of 'called' is 'd'.
'esp'+'ie'+'d'='ESPIED'
(Other definitions for espied that I've seen before include "Glimpsed" , "Caught sight of (archaic)" , "Watched" , "Spotted; saw" , "observed in literature" .)