Fellow mariners initially hand over navigational device (7)
I believe the answer is:
compass
'navigational device' is the definition.
(instrument which helps with navigation)
'fellow mariners initially hand' is the wordplay.
'fellow' becomes 'co' (eg a co-worker is a fellow worker).
'initially' suggests taking the first letters.
'hand' becomes 'pass' (synonyms).
The initial letter of 'mariners' is 'm'.
'co'+'m'+'pass'='COMPASS'
'over' acts as a link.
(Other definitions for compass that I've seen before include "Range of pitch" , "Direction-finding instrument" , "Instrument for determining direction from magnetic needle" , "Direction finding instrument - range" , "Range, scope" .)