Rock plant's last seen by aquatic creature (6)
I believe the answer is:
totter
'rock' is the definition.
(I've seen this in another clue)
'plant's last seen by aquatic creature' is the wordplay.
'last' suggests the final letters.
'seen by' means one lot of letters go next to another.
'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" .)