Nearly all ships beat a retreat (4)
I believe the answer is:
flee
'beat a retreat' is the definition.
(I've seen this in another clue)
'nearly all ships' is the wordplay.
'nearly all' means to remove the last letter (most but not all of the word).
'ships' becomes 'fleet' (eg a naval fleet).
'fleet' with its final letter removed is 'FLEE'.
(Other definitions for flee that I've seen before include "Do a runner" , "Run (from danger)" , "Turn tail" , "retreat" , "Take flight" .)