Put one's foot down in the introduction (4)
I believe the answer is:
trod
'put one's foot down' is the definition.
(I've seen this in another clue)
'in the introduction' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TROD' can be found hidden inside 'introduction'.
(Other definitions for trod that I've seen before include "Pressed (grapes) by foot" , "Crushed (grapes) by foot" , "Walked (on something)" , "Pressed (something) down with the feet" , "Took steps" .)