Lab users concealing cruel treatment (5)
I believe the answer is:
abuse
'cruel treatment' is the definition.
(I've seen this before)
'lab users concealing' is the wordplay.
'concealing' indicates the answer is hidden within the clue.
'ABUSE' can be found hidden inside 'lab users'.
(Other definitions for abuse that I've seen before include "corrupt practice" , "Treat with cruelty" , "Address insultingly" , "Mistreatment" , "Castigate" .)