Submariner beginning to navigate through twisted ocean plants (8)
I believe the answer is:
anemones
'plants' is the definition.
(anemone is a kind of plant)
'submariner beginning to navigate through twisted ocean' is the wordplay.
'submariner' becomes 'Nemo' (Captain Nemo in 20,000 Leagues Under the Sea).
'beginning to' indicates taking the first letters.
'through' is an insertion indicator (inserted letters go through the word).
'twisted' says the letters should be written backwards.
'ocean' becomes 'sea' (synonyms).
The first letter of 'navigate' is 'n'.
'sea' reversed gives 'aes'.
'nemo'+'n'='nemon'
'nemon' placed within 'aes' is 'ANEMONES'.
(Other definitions for anemones that I've seen before include "Mean nose (anag.)" , "Sane omen for sea creatures" , "bloomers" , "No enemas for these flowers" , "Plants" .)