The Arabic girl, perhaps, describes aquatic plant (5)
I believe the answer is:
algal
'describes aquatic plant' is the definition.
I can't judge whether this definition defines the answer.
'the arabic girl' is the wordplay.
'the arabic' becomes 'al' (I can't explain this - if you can you should believe this answer much more).
'girl' becomes 'gal'.
'al'+'gal'='ALGAL'
'perhaps' is the link.
Can you help me to learn more?
(Other definitions for algal that I've seen before include "Relating to seaweeds, etc" , "Pertaining to such as seaweeds" .)