Sailors mostly knowledgeable about hot drink? (7)
I believe the answer is:
absinth
'drink?' is the definition.
(absinth is a kind of drink)
'sailors mostly knowledgeable about hot' is the wordplay.
'sailors' becomes 'abs' (AB is short for able seaman).
'mostly' means to remove the last letter (most but not all of the word).
'knowledgeable about' becomes 'into' (I've seen this before).
'hot' becomes 'h' (abbreviation eg on taps).
'into' with its final letter removed is 'int'.
'abs'+'int'+'h'='ABSINTH'
(Other definitions for absinth that I've seen before include "(Originally) a wormwood-based liqueur" , "Aniseed-flavoured liqueur" , "Strong green alcoholic drink" , "Strong liquor in baths, strangely" , "sort of liqueur" .)