On walk lightning's ultimate risk (6)
I believe the answer is:
gamble
'risk' is the definition.
(gamble is a kind of risk)
'on walk lightning's ultimate' is the wordplay.
'on' says to put letters next to each other.
'walk' becomes 'amble' (amble is a kind of walk**).
'ultimate' indicates one should take the final letters.
The last letter of 'lightning' is 'g'.
'amble' put after 'g' is 'GAMBLE'.
(Other definitions for gamble that I've seen before include "Take a risk" , "Take a chance, speculate" , "Wager; sounds like frolic" , "Run a risk" , "Bet money" .)