Join teaching assistant in brook (9)
I believe the answer is:
entertain
'brook' is the definition.
The answer and definition can be both related to thinking as well as being verbs in their base form.
Perhaps they are linked in a way I don't understand?
'join teaching assistant in' is the wordplay.
'join' becomes 'enter' (I've seen this in another clue).
'teaching assistant' becomes 'TA' (employee in UK school).
'enter'+'ta'+'in'='ENTERTAIN'
Can you help me to learn more?
(Other definitions for entertain that I've seen before include "Wine and dine" , "Amuse or take into consideration" , "Amuse, divert" , "Train teen to amuse" , "Amuse - give hospitality" .)