First to arrive discovers cruel destruction (6)
I believe the answer is:
eldest
'first to arrive' is the definition.
(I've seen this in another clue)
'discovers cruel destruction' is the wordplay.
'discovers' indicates the answer is hidden within the clue (I've seen 'discovered' mean this (remove the letters covering the answer in the clue)).
'ELDEST' is hidden within 'cruel destruction'.
(Other definitions for eldest that I've seen before include "Most senior of two" , "First in the order of birth" , "Of greatest age" , "First to be born" , "heir?" .)