People in charge at sea must get butterflies (8)
I believe the answer is:
admirals
This clue is a double definition.
'people in charge at sea' is the first definition.
(senior naval officers)
'butterflies' is the second definition.
(admiral is a kind of butterfly)
'must get' acts as a link.
(Other definitions for admirals that I've seen before include "Supreme commanders of fleets" , "Officers" , "Supreme fleet commanders" .)