Rival work enclosed working parts (8)
I believe the answer is:
opponent
'rival' is the definition.
(a rival is an opponent or enemy)
'work enclosed working parts' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'enclosed' becomes 'pent' (I've seen this in another clue).
'working' becomes 'on' (on can mean working or activated).
'parts' is an insertion indicator (one word creates a gap in ie parts another word).
'pent' going around 'on' is 'ponent'.
'op'+'ponent'='OPPONENT'
(Other definitions for opponent that I've seen before include "One competes against you" , "Adversary, competitor" , "Foe or competitor" , "member of other team" , "One's competitor" .)