Lenses, etc., lost in the post, I see (6)
I believe the answer is:
optics
'lenses etc lost' is the definition.
I can't tell whether this defines the answer.
'in the post i see' is the wordplay.
'in the' indicates putting letters inside.
'post' becomes 'opts' (I am not sure about this - if you are sure you should believe this answer much more).
'see' becomes 'C' (the word for the letter, according to Chambers).
'i'+'c'='ic'
'opts' going around 'ic' is 'OPTICS'.
Can you help me to learn more?
(Other definitions for optics that I've seen before include "Spirit measures" , "The scientific study of light and vision" , "Scientific studies of light and vision, the eyes" , "Science concerned with vision" , "Drink measuring devices" .)