March hares essentially overcome by swirling tides (6)
I believe the answer is:
stride
'march' is the definition.
(synonyms)
'hares essentially overcome by swirling tides' is the wordplay.
'essentially' says to take the centre.
'overcome by' means one lot of letters goes inside another.
'swirling' indicates an anagram.
The middle letter of 'hares' is 'r'.
'tides' is an anagram of 'stide'.
'r' going inside 'stide' is 'STRIDE'.
(Other definitions for stride that I've seen before include "use your legs!" , "jazz piano-playing" , "A lengthy step" , "Long step of direst kind" , "Long walking step" .)