At end, it became corrupted (7)
I believe the answer is:
tainted
'corrupted' is the definition.
('taint' can be a synonym of 'corrupt')
'at end it became' is the wordplay.
'became' indicates an anagram (I've seen 'becomes' mean this).
'at'+'end'+'it'='atendit'
'atendit' with letters rearranged gives 'TAINTED'.
(Other definitions for tainted that I've seen before include "Corrupted, sullied" , "Spoiled, stained" , "Contaminated, polluted" , "Flawed, contaminated" , "corrupt" .)