Perform in practice (3)
I believe the answer is:
act
'perform' is the definition.
(acting is a kind of performing)
'in practice' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ACT' is hidden within 'practice'.
(Other definitions for act that I've seen before include "Parliamentary decree" , "Produce an effect" , "Major section of a play" , "Function" , "law passed" .)