Initial ship gone, every other one leaving (4)
I believe the answer is:
sign
'initial' is the definition.
(thesaurus)
'ship gone every other one leaving' is the wordplay.
'every other one leaving' means one should take alternating letters (every other letter removed).
'ship'+'gone'='shipgone'
The alternate letters of 'shipgone' are 'SIGN'.
(Other definitions for sign that I've seen before include "Token - symbol - omen - write one's name" , "Notice; symbol" , "Traffic control - to write one's name" , "Show" , "Evidence" .)