This slippery type left one alone finally going west (3)
I believe the answer is:
eel
'this slippery type' is the definition.
(I've seen this before)
'left one alone finally going west' is the wordplay.
'left' becomes 'L' (common abbreviation).
'finally' says to take the final letters.
'going west' is a reversal indicator.
The last letters of 'one alone' are 'ee'.
'l'+'ee'='lee'
'lee' reversed gives 'EEL'.
(Other definitions for eel that I've seen before include "Sea creature like conger" , "Snake-like sea creature" , "Long smooth fish" , "Fish in the Lee?" , "Conger, e.g." .)