Ponder gearwheel — it corroded (8)
I believe the answer is:
cogitate
'ponder' is the definition.
(I know that ponder is a more specific form of the action cogitate)
'gearwheel it corroded' is the wordplay.
'gearwheel' becomes 'cog' (I've seen this in another clue).
'corroded' becomes 'ate' ('eat' can be a synonym of 'corrode').
'cog'+'it'+'ate'='COGITATE'
(Other definitions for cogitate that I've seen before include "Think deeply, ponder" , "Ruminate" , "Use little grey cells" , "Reflect, ponder" , "Do some thinking" .)