Mishandle a tiller, sadly leading to end of boat (3-5)
I believe the answer is:
ill-treat
'mishandle' is the definition.
(thesaurus)
'a tiller sadly leading to end of boat' is the wordplay.
'sadly' indicates anagramming the letters.
'leading to' is a charade indicator (letters next to each other).
'end of' indicates one should take the final letters.
The last letter of 'boat' is 't'.
'tiller' with letters rearranged gives 'illtre'.
'a' after 'illtre' is 'illtrea'.
'illtrea'+'t'='ILL-TREAT'
(Other definitions for ill-treat that I've seen before include "Subject to cruel or abusive usage" , "Misuse" , "Abuse" , "Use someone badly or cruelly" , "Act cruelly towards (someone)" .)