Amphitheatre is part of warfare, naturally (5)
I believe the answer is:
arena
'amphitheatre' is the definition.
(I've seen this before)
'part of warfare naturally' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ARENA' is hidden in the letters of 'warfare naturally'.
'is' acts as a link.
(Other definitions for arena that I've seen before include "Venue for boxing, circus and other action events" , "Central area in sports stadium" , "Place for public events" , "... might perform here" , "centre of conflict" .)