Fight fire in carrier at the back (8)

I believe the answer is:
rucksack
'carrier at the back' is the definition.
I can't tell whether this definition defines the answer.
'fight fire' is the wordplay.
'fight' becomes 'ruck' (I've seen this before).
'fire' becomes 'sack' (dismiss someone from their job).
'ruck'+'sack'='RUCKSACK'
'in' is the link.
Can you help me to learn more?
(Other definitions for rucksack that I've seen before include "A hiker's backpack carrier" , "large bag" , "Back carrier" , "Hiker often carries one on back" , "part of hiker's gear" .)
