Djokovic, for one, involved in tenser battle (4)
I believe the answer is:
serb
'djokovic for one' is the definition.
(I've seen this in another clue)
'involved in tenser battle' is the wordplay.
'involved in' says the answer is hidden in the clue.
'SERB' is hidden within 'tenser battle'.
(Other definitions for serb that I've seen before include "Balkan native" , "A European national" , "Native of the Balkans" , "Eastern European" , "Eg, native of Belgrade" .)