Implement brought back in Waterloo train (4)
I believe the answer is:
tool
'implement' is the definition.
(I know that implement can be written as tool)
'brought back in waterloo train' is the wordplay.
'brought back' says the letters should be written in reverse.
'in' indicates a hidden word.
'loot' is hidden within 'waterloo train'.
'loot' reversed gives 'TOOL'.
(Other definitions for tool that I've seen before include "Weapon" , "Maybe saw" , "Cat's-paw; impress design on" , "Working instrument" , "For example, saw" .)