Cruel way to get the heart of the truth in the lab, perhaps (6)
I believe the answer is:
brutal
'cruel' is the definition.
(I know that cruel can be written as brutal)
'the heart of the truth in the lab perhaps' is the wordplay.
'the heart of' says to take the centre.
'in the' is an insertion indicator.
'perhaps' indicates anagramming the letters.
The middle of 'truth' is 'rut'.
'lab' anagrammed gives 'bal'.
'rut' placed into 'bal' is 'BRUTAL'.
'way to get' acts as a link.
(Other definitions for brutal that I've seen before include "Rottweiler" , "Cruel, in animal fashion" , "Savagely violent" , "Inhuman - pitiless" , "Disposed to inflict pain" .)