Sacrifice one companion crossing second lake (8)
I believe the answer is:
immolate
'sacrifice' is the definition.
(I know that immolate is a more specific form of the action sacrifice)
'one companion crossing second lake' is the wordplay.
'one' becomes 'i' (Roman numeral).
'companion' becomes 'mate' (both can mean a friend).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'second' becomes 'mo' (both are short periods of time).
'lake' becomes 'L' (geographical abbreviation).
'mo'+'l'='mol'
'mate' placed around 'mol' is 'mmolate'.
'i'+'mmolate'='IMMOLATE'
(Other definitions for immolate that I've seen before include "Destroy; burn" , "Sacrifice, kill or destroy" , "Offer as sacrifice (usually by burning)" , "Limo team involved in sacrifice" .)