Seas that Armada tours, going round the bend (2,3,2,1,6)
I believe the answer is:
as mad as a hatter
'going round the bend' is the definition.
'seas that armada tours' is the wordplay.
'tours' indicates an anagram (the letters roam or tour around).
'seas'+'that'+'armada'='seasthatarmada'
'seasthatarmada' is an anagram of 'AS MAD AS A HATTER'.
(Other definitions for as mad as a hatter that I've seen before include "Troubled, as one responsible for tiles" , "having a tile loose?" .)