Put your foot down in second and first (6)
I believe the answer is:
insist
'put your foot down' is the definition.
(I've seen this before)
'in second and first' is the wordplay.
'second' becomes 's' (common abbreviation).
'and' says to put letters next to each other.
'first' becomes 'ist' (ist resembles 1st).
'in'+'s'+'ist'='INSIST'
(Other definitions for insist that I've seen before include "Press" , "'Demand, make sure (6)'" , "Demand firmly" , "Beg persistently and urgently" , "Be adamant" .)