What water pipes should be kept behind? (6)
I believe the answer is:
lagged
This clue is a double definition.
'what water pipes should be' is the first definition.
(to lag is to cover pipes in insulating material)
'kept behind?' is the second definition.
(Other definitions for lagged that I've seen before include "Got behind; insulated" , "Fell behind others" , "Fell behind; insulated" , "insurance against freezing" , "Wrapped; fell behind" .)