Cruel treatment sailor almost used (5)
I believe the answer is:
abuse
'cruel treatment' is the definition.
(I've seen this before)
'sailor almost used' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'almost' means to remove the last letter (most of the word but not all of it).
'used' with its final letter taken away is 'use'.
'ab'+'use'='ABUSE'
(Other definitions for abuse that I've seen before include "Deceit" , "Misemploy - invective" , "Mistreatment" , "Treat badly - hurl insults" , "Maltreat" .)