Load intercepted by English lieutenant remains in vessel (7)
I believe the answer is:
'remains in vessel' is the definition.
(amount of alcohol at the bottom of a glass)
'load intercepted by english lieutenant' is the wordplay.
'load' becomes 'heap' (I've seen this in another clue).
'intercepted by' means one lot of letters goes inside another (I've seen this in other clues).
'english' becomes 'e' (abbreviation).
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'heap' placed around 'elt' is 'HEELTAP'.
(Other definitions for heeltap that I've seen before include "Remains of drink in glass", "little left to drink", "Shoe-layer material; dregs in glass", "Layer of leather at the back of a shoe", "Drop left in glass".)