Nitrogen close to ocean absorbed by marine plant (5)
I believe the answer is:
senna
'plant' is the definition.
The answer is an example.
'nitrogen close to ocean absorbed by marine' is the wordplay.
'nitrogen' becomes 'n' (N is the chemical symbol for nitrogen).
'close to' indicates one should take the final letters (the close/ending of).
'absorbed by' means one lot of letters goes inside another.
'marine' becomes 'sea' (both can mean 'of the sea').
The last letter of 'ocean' is 'n'.
'n'+'n'='nn'
'nn' going into 'sea' is 'SENNA'.
(Other definitions for senna that I've seen before include "He drove fast" , "should make you want to go quickly" , "Champion driver.once" , "Formula One driver" , "Plant with medicinal pods" .)