Behave in practice (3)
I believe the answer is:
act
'behave' is the definition.
(acting is a kind of behaving)
'in practice' is the wordplay.
'in' says the answer is hidden in the clue.
'ACT' is hidden in the letters of 'practice'.
(Other definitions for act that I've seen before include "turn" , "Perform theatrically" , "Major section of a play" , "Behave like thespian" , "Exploit" .)