Shortly, Samuel will welcome Sidney back for painful practice (6)
I believe the answer is:
sadism
'painful practice' is the definition.
(deriving pleasure from inflicting pain)
'shortly samuel will welcome sidney back' is the wordplay.
'shortly samuel' becomes 'Sam' (Sam is short for Samuel).
'will welcome' indicates putting letters inside.
'sidney' becomes 'Sid' (Sid is short for Sidney).
'back' is a reversal indicator.
'sid' backwards is 'dis'.
'sam' enclosing 'dis' is 'SADISM'.
'for' is the link.
(Other definitions for sadism that I've seen before include "Sexual pleasure from hurting others" , "Dim ass (anag)" , "Deviancy" , "Enjoyment taken in inflicting pain or cruelty" , "Deliberate cruelty" .)