Twain book about sailor on whiskey (7)
I believe the answer is:
twosome
'twain' is the definition.
(I know that twain can be written as twosome)
'book about sailor on whiskey' is the wordplay.
'book' becomes 'tome' (a tome is a book).
'about' is an insertion indicator.
'sailor' becomes 'os' (ordinary seaman).
'on' is a charade indicator (letters next to each other).
'whiskey' becomes 'W' (phonetic alphabet: alpha, bravo, charlie etc.).
'os' after 'w' is 'wos'.
'tome' enclosing 'wos' is 'TWOSOME'.
(Other definitions for twosome that I've seen before include "Pair, duo" , "Item" , "match" , "game of golf" , "Couple" .)