Underwater mud grebes churned up (9)
I believe the answer is:
submerged
'underwater' is the definition.
('submerged' can be a synonym of 'underwater')
'mud grebes churned up' is the wordplay.
'churned up' indicates anagramming the letters.
'mud'+'grebes'='mudgrebes'
'mudgrebes' with letters rearranged gives 'SUBMERGED'.
(Other definitions for submerged that I've seen before include "overwhelmed" , "Completely covered by liquid" , "Under water" .)