Chastise unit at sea that's keen (12)
I believe the answer is:
enthusiastic
'keen' is the definition.
(thesaurus)
'chastise unit at sea' is the wordplay.
'at sea' is an anagram indicator.
'chastise'+'unit'='chastiseunit'
'chastiseunit' with letters rearranged gives 'ENTHUSIASTIC'.
'that's' acts as a link.
(Other definitions for enthusiastic that I've seen before include "Full of fervour" , "Ardent, zealous" , "Gung-ho" , "warm" , "Showing keen interest" .)