Stop sailor treating rot (5)
I believe the answer is:
abort
'stop' is the definition.
(I've seen this before)
'sailor treating rot' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'treating' is an anagram indicator.
'rot' with letters rearranged gives 'ort'.
'ab'+'ort'='ABORT'
(Other definitions for abort that I've seen before include "Bring to premature conclusion" , "Terminate prematurely" , "Miscarry" , "End prematurely" , "Stop prematurely" .)