Powerful rescue vehicle is a great help (5,2,8)
I believe the answer is:
tower of strength
'great help' is the definition.
(tower of strength means a very helpful person)
I don't understand the rest of the clue.
(Other definitions for tower of strength that I've seen before include "Powerful tractor" , "True supporter" , "Person giving support" , "one pulling a lorry by rope?" , "person to be relied upon" .)