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' indicates the answer is hidden within the clue (the clue goes around or about the answer).
'INK' is hidden within '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" .)