Advises Nelson on reflection to carry glasses to help him see better, perhaps (6)
I believe the answer is:
lenses
'glasses to help him see better perhaps' is the definition.
The definition suggests a singular noun but the answer is a plural noun.
'advises nelson on reflection to carry' is the wordplay.
'to carry' says the answer is hidden in the clue (I've seen 'carrying' mean this).
'on reflection' shows that the letters should be reversed in order.
'sesnel' is hidden within 'advises nelson'.
'sesnel' back-to-front is 'LENSES'.
Can you help me to learn more?
(Other definitions for lenses that I've seen before include "They're transparent in the eye" , "Focussing glasses" , "Camera attachments" , "They're transparent in the eyes" , "Optical elements" .)