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