Single plant acquires nitrogen (5)
I believe the answer is:
alone
'single' is the definition.
(synonyms)
'plant acquires nitrogen' is the wordplay.
'plant' becomes 'aloe' (I can't justify this - if you can you should believe this answer much more).
'acquires' indicates putting letters inside.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'aloe' enclosing 'n' is 'ALONE'.
Can you help me to learn more?
(Other definitions for alone that I've seen before include "Singly" , "Single" , "Isolated; exclusively" , "Lacking company, solitary" , "On one's own" .)