Behind some of the master navigators (6)
I believe the answer is:
astern
'behind' is the definition.
(at the back or behind of a ship)
'some of the master navigators' is the wordplay.
'some of' indicates a hidden word.
'ASTERN' can be found hidden inside 'master navigators'.
(Other definitions for astern that I've seen before include "Behind (a boat)" , "Towards the rear of boat" , "Aft" , "Ran set (anag.)" , "Towards a ship's rear" .)