Aid rebels at sea? That's sensible (9)
I believe the answer is:
desirable
'sensible' is the definition.
'aid rebels at sea?' is the wordplay.
'at sea?' indicates anagramming the letters (I've seen 'at sea' mean this).
'aid'+'rebels'='aidrebels'
'aidrebels' with letters rearranged gives 'DESIRABLE'.
'that's' acts as a link.
(Other definitions for desirable that I've seen before include "Pleasing" , "Alluring" , "Worth having" , "handsome maybe" , "Sought-after - rebel aids (anag)" .)