Eviscerate revolting snake at bottom of mast in towing vessel (7)
I believe the answer is:
tugboat
'towing vessel' is the definition.
(I've seen this before)
'eviscerate revolting snake at bottom of mast' is the wordplay.
'eviscerate' becomes 'gut' (I've seen this before).
'revolting' says the letters should be written backwards.
'snake' becomes 'boa' (boa is a kind of snake).
'at' is a charade indicator (letters next to each other).
'bottom of' suggests the final letters.
The last letter of 'mast' is 't'.
'gut' back-to-front is 'tug'.
'tug'+'boa'+'t'='TUGBOAT'
'in' is the link.
(Other definitions for tugboat that I've seen before include "Drawing craft" , "Small vessel pulling a big one" , "Vessel drawing" , "Harbour craft" , "Ship pulling a larger one" .)