If all plants were this, there would be no great variety (5)
I believe the answer is:
dwarf
'if all plants were this there' is the definition.
The definition suggests a singular noun which matches the answer.
'would be no great variety' is the wordplay.
'would' becomes 'd' (I've seen this in other clues).
'be no great variety' becomes 'warf' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'd'+'warf'='DWARF'
Can you help me to learn more?
(Other definitions for dwarf that I've seen before include "One of the Snow White Seven?" , "Stunt the growth of" , "Grumpy, e.g." , "Overshadow" , "Happy for one" .)