Heartless, silly to stop practice (6)
I believe the answer is:
system
'practice' is the definition.
(thesaurus)
'heartless silly to stop' is the wordplay.
'heartless' suggests removing the centre.
'to stop' becomes 'stem' (stemming is a kind of stopping).
'silly' with its centre removed is 'sy'.
'sy'+'stem'='SYSTEM'
(Other definitions for system that I've seen before include "set-up" , "Arrangement of parts functioning together" , "Group of elements forming a unified whole" , "Organised means of work" , "Method (of betters?)" .)