More manageable at sea; at sea? That’s right (6)
I believe the answer is:
easier
'more manageable' is the definition.
(easier to deal with or manage)
'sea at sea? that's right' is the wordplay.
'at sea?' is an anagram indicator (I've seen 'at sea' mean this).
'that's' becomes 'ie'.
'right' becomes 'R' (common abbreviation).
'sea' anagrammed gives 'eas'.
'eas'+'ie'+'r'='EASIER'
'at' acts as a link.
(Other definitions for easier that I've seen before include "more like a piece of cake" , "More effortless" , "Simpler" , "Less taxing" , "Requiring less effort" .)