It's nailed on the foot (3)
I believe the answer is:
toe
'it's nailed' is the definition.
(I've seen this in another clue)
'on the foot' is the wordplay.
'on' becomes 'to'.
'foot' says to take the final letters (in a down clue, the bottom letter).
The final letter of 'the' is 'e'.
'to'+'e'='TOE'
(Other definitions for toe that I've seen before include "Extremity of the body" , "One with sock on" , "Pedal digit" , "compliantly put on line" , "Small member" .)