Special circle linked, they say, to period of high water (6,4)
I believe the answer is:
spring tide
'period of high water' is the definition.
'special circle linked they say' is the wordplay.
'special' becomes 'sp'.
'circle' becomes 'ring' ('ring' can be a synonym of 'circle').
'linked' becomes 'tied' (tying is a kind of linking).
'they say' shows a homophone (sound like).
'tied' sounds like 'tide'.
'sp'+'ring'+'tide'='SPRING-TIDE'
'to' acts as a link.
(Other definitions for spring tide that I've seen before include "Dregs in pit (anag.)" , "High water" , "When rise and fall of the sea is greatest" , "It occurs at new and full moon" , "It is linked to full and new moon" .)