Go green at sea on top of yardarm (6)
I believe the answer is:
energy
'go' is the definition.
(thesaurus)
'green at sea on top of yardarm' is the wordplay.
'at sea' indicates anagramming the letters.
'on top of' suggests taking the first letters (I've seen 'on top' mean this).
The first letter of 'yardarm' is 'y'.
'green' anagrammed gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Vigour, spirit" , "Capacity for action" , "Go green, possibly with envy, ultimately" , "life" , "Vigour, power" .)