Small key for mariner (4)
I believe the answer is:
salt
'for mariner' is the definition.
(synonyms)
'small key' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'key' becomes 'alt' (the Alt keyboard button).
's'+'alt'='SALT'
(Other definitions for salt that I've seen before include "Preserve" , "Seasoned sailor" , "Condiment - old sailor" , "Condiment - experienced sailor" , "Mariner; sodium chloride" .)