Leaders of naval expedition are reconnoitring sea approaches (5)
I believe the answer is:
nears
'approaches' is the definition.
('near' can be a synonym of 'approach')
'leaders of naval expedition are reconnoitring sea' is the wordplay.
'leaders of' suggests taking the first letters.
The first letters of 'naval expedition are reconnoitring sea' is 'NEARS'.
(Other definitions for nears that I've seen before include "Moves towards" , "Becomes closer" , "Draws close" , "Uses snare to get closer" , "Approaches - snare (anag)" .)