Coming up, mouths moved through the water (4)
I believe the answer is:
swam
'moved through the water' is the definition.
(I've seen this before)
'coming up mouths' is the wordplay.
'coming up' says the letters should be written in reverse (in a down clue, letters go up).
'mouths' becomes 'maws' (maw is a kind of mouth).
'maws' written backwards gives 'SWAM'.
(Other definitions for swam that I've seen before include "Travelled through water" , "Moved through water" , "Did butterfly perhaps" , "was dizzy" , "Splashed around" .)