Thinking about writing material (3)
I believe the answer is:
ink
'writing material' is the definition.
(substance for writing with)
'thinking about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'INK' can be found hidden inside 'thinking'.
(Other definitions for ink that I've seen before include "Fluid from cuttlefish" , "Printer's medium" , "Liquid used to write" , "Writing medium" , "Cuttlefish defence" .)