Couple in one sea weakened (8)
I believe the answer is:
impaired
'weakened' is the definition.
(similar in meaning)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for impaired that I've seen before include "Spoiled" , "Caused weakening, damaged" , "Spoilt" , "Had negative impact" , "Defective" .)