Go off on one extremely large excursion (3,3)
I believe the answer is:
let rip
'go off on one' is the definition.
(both can mean to demonstrate intense anger)
'extremely large excursion' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'excursion' becomes 'trip' (I've seen this before).
'large' with its centre removed is 'le'.
'le'+'trip'='LET RIP'
(Other definitions for let rip that I've seen before include "Go wild" , "Hasten" , "Release pent-up feelings" , "Triple (anag)" , "Sound off" .)