Stop sailor spreading rot (5)
I believe the answer is:
abort
'stop' is the definition.
(as in aborting a mission)
'sailor spreading rot' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'spreading' indicates anagramming the letters.
'rot' anagrammed gives 'ort'.
'ab'+'ort'='ABORT'
(Other definitions for abort that I've seen before include "Miscarry" , "Bring to premature conclusion" , "Pull the plug on" , "Terminate prematurely" , "End prematurely" .)