Rock plant's last seen by aquatic creature (6)
I believe the answer is:
totter
'rock' is the definition.
(I know that totter is a more specific form of the action rock)
'plant's last seen by aquatic creature' is the wordplay.
'last' suggests the final letters.
'seen by' is a charade indicator (letters next to each other).
'aquatic creature' becomes 'otter' (animal that lives by water).
The final letter of 'plant' is 't'.
't'+'otter'='TOTTER'
(Other definitions for totter that I've seen before include "Rag-and-bone man - summer" , "I add" , "Maybe a mathematician" , "Walk shakily as if about to fall" , "Move unsteadily; be about to fall" .)