Green from mal de mer , miles out at sea (7)
I believe the answer is:
emerald
'green' is the definition.
(emerald is a shade of green)
'mal de mer miles out at sea' is the wordplay.
'miles' becomes 'm' (common abbreviation).
'out' suggests deleting specific letters.
'at sea' indicates an anagram.
'mal'+'de'+'mer'='maldemer'
'maldemer' with 'm' removed is 'aldemer'.
'aldemer' with letters rearranged gives 'EMERALD'.
'from' acts as a link.
(Other definitions for emerald that I've seen before include "On of the Greens" , "Highly-valued variety of beryl" , "Precious gemstone" , "Green - jewel" , "Shade of green" .)