A miner all at sea in saltwater (6)
I believe the answer is:
marine
'saltwater' is the definition.
(synonyms)
'a miner all at sea in' is the wordplay.
'all at sea' indicates an anagram.
'in' is an insertion indicator.
'miner' with letters rearranged gives 'mrine'.
'a' placed into 'mrine' is 'MARINE'.
(Other definitions for marine that I've seen before include "Soldier serving on land and sea" , "Maritime - seagoing soldier" , "Relating to shipping or naval matters" , "Soldier associated with a navy" , "Soldier-sailor" .)