On top shelf perhaps, thinner pen (11)
I believe the answer is:
highlighter
'pen' is the definition.
(I know that highlighter is a type of felt-tip pen)
'on top shelf perhaps thinner' is the wordplay.
'on' is a charade indicator (letters next to each other).
'top shelf perhaps' becomes 'high' (I can't justify this - if you can you should give a lot more credence to this answer).
'thinner' becomes 'lighter' ('light' can be similar in meaning to 'thin').
'high' put next to 'lighter' is 'HIGHLIGHTER'.
Can you help me to learn more?
(Other definitions for highlighter that I've seen before include "Bright marker pen" , "Fluorescent pen used to mark parts of a text - the high girl (anag)" , "Pen marking route" , "Pen for emphasising text" .)