Tension causes rest to be disturbed in the ship (6)
I believe the answer is:
stress
'tension' is the definition.
(synonyms)
'rest to be disturbed in the ship' is the wordplay.
'to be disturbed' is an anagram indicator (the order of letters is disturbed).
'in the' indicates putting letters inside.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'rest' with letters rearranged gives 'tres'.
'tres' inserted inside 'ss' is 'STRESS'.
'causes' is the link.
(Other definitions for stress that I've seen before include "Pressure or strain" , "suffering" , "Emphasize" , "Emotional tension" , "possible excuse for sickie?" .)