Second lieutenant locking up a seasoned sailor (4)
I believe the answer is:
salt
'seasoned sailor' is the definition.
(I've seen this before)
'second lieutenant locking up a' is the wordplay.
'second' becomes 's' (common abbreviation**).
'lieutenant' becomes 'lt'.
'locking up' means one lot of letters goes inside another (I've seen this in other clues).
's'+'lt'='slt'
'slt' placed around 'a' is 'SALT'.
(Other definitions for salt that I've seen before include "Liveliness" , "Brackish" , "A defence against ice" , "Pile up" , "Crystalline substance for the table" .)