Be alone when Ben has left the plant (4)
I believe the answer is:
aloe
'the plant' is the definition.
(aloe is a kind of plant)
'be alone when ben has left' is the wordplay.
I cannot quite see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
'left' could be 'al' (I've seen this in other clues) and 'al' is present in the answer.
A single letter 'o' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for aloe that I've seen before include "Flower, maybe vera" , "Fleshy-leaved plant" , "Genus of bitter herbs" , "Bitter drug from a leaf juice" , "Plant source of purgative drug" .)