Sailor posed outside front of library (4)
I believe the answer is:
salt
'sailor' is the definition.
(salt is a kind of sailor)
'posed outside front of library' is the wordplay.
'posed' becomes 'sat' (sitting is a kind of posing).
'outside' means one lot of letters goes inside another.
'front of' suggests taking the first letters.
The first letter of 'library' is 'l'.
'sat' enclosing 'l' is 'SALT'.
(Other definitions for salt that I've seen before include "Preserve" , "Wit; sodium chloride" , "A defence against ice" , "NaCl" , "Preservative" .)