Marking helping to support pupil once (9)
I believe the answer is:
observing
'marking' is the definition.
The definition and answer can be both related to thinking as well as being present participle verbs.
Perhaps you can see an association between them that I can't see?
'helping to support pupil once' is the wordplay.
'helping' becomes 'serving' (a helping is a serving of food).
'to support' means one lot of letters go next to another (in a down clue, some letters hold others up).
'pupil once' becomes 'ob'.
'serving' put after 'ob' is 'OBSERVING'.
Can you help me to learn more?
(Other definitions for observing that I've seen before include "Looking at" , "Taking note of" , "Watching attentively" , "Noticing or perceiving" , "Watching and perhaps taking notes" .)