Watchful navigator holds a bone (4)
I believe the answer is:
ulna
'a bone' is the definition.
(I know that ulna is a bone)
'watchful navigator holds' is the wordplay.
'holds' indicates the answer is hidden within the clue.
'ULNA' is hidden in the letters of 'watchful navigator'.
(Other definitions for ulna that I've seen before include "part of anatomy" , "The longer and inner bone in forearm" , "Thinner and longer of two bones in forearm" , "part of arm" , "Car document; ticket book" .)