Strain shown by rest at sea on steamship (6)
I believe the answer is:
stress
'strain' is the definition.
('stress' can be a synonym of 'strain')
'rest at sea on steamship' is the wordplay.
'at sea' indicates anagramming the letters.
'on' is an insertion indicator.
'steamship' becomes 'ss'.
'rest' anagrammed gives 'tres'.
'tres' put inside 'ss' is 'STRESS'.
'shown by' acts as a link.
(Other definitions for stress that I've seen before include "Emphasize" , "Emphasise the strain" , "'Strain, tension (6)'" , "nervous tension" , "Dwell on" .)