Implausibly fast mover on yacht at sea (7)
I believe the answer is:
'implausibly fast mover' is the definition.
(hypothetical faster-than-light particle)
'on yacht at sea' is the wordplay.
'at sea' indicates an anagram.
'onyacht' is an anagram of 'TACHYON'.
(Another definition for tachyon that I've seen is " One moving faster than light".)