Author entertaining old student left plant powder (6)
I believe the answer is:
pollen
'plant powder' is the definition.
'pollen' can be an answer for 'powder' (pollen is a kind of powder). I am unsure of the 'plant' bit.
'author entertaining old student left' is the wordplay.
'author' becomes 'pen' (to pen something is to author it).
'entertaining' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'student' becomes 'l' (as in L-plates for learner drivers).
'left' becomes 'l' (common abbreviation).
'o'+'l'+'l'='oll'
'pen' going around 'oll' is 'POLLEN'.
Can you help me to learn more?
(Other definitions for pollen that I've seen before include "Flower powder" , "Dust-like powder from some plants" , "Fertilising dust from flower" , "Airborne 10 across" , "Powdery substance given off by plants" .)