Dishearten sailor and deem unfit (10)
I believe the answer is:
demoralise
'dishearten' is the definition.
('dishearten'->'demoralize' is in my internal thesaurus and 'ize'->'ise')
'sailor and deem unfit' is the wordplay.
'and' means one lot of letters go next to another.
'unfit' indicates anagramming the letters.
'sailor'+'deem'='sailordeem'
'sailordeem' is an anagram of 'DEMORALISE'.
(Other definitions for demoralise that I've seen before include "Make downhearted" , "Cause to lose confidence" , "Crush" , "Cause to lose confidence/hope" , "Throw into confusion" .)