Fiend runs into position for protest (13)
I believe the answer is:
demonstration
'protest' is the definition.
(I've seen this before)
'fiend runs into position' is the wordplay.
'fiend' becomes 'demon' (I've seen this before).
'runs' becomes 'r' (cricket abbreviation).
'into' means one lot of letters goes inside another.
'position' becomes 'station' (station is a kind of position**).
'r' going inside 'station' is 'stration'.
'demon'+'stration'='DEMONSTRATION'
'for' acts as a link.
(Other definitions for demonstration that I've seen before include "Exhibition, protest" , "Protest meeting" , "Protest march/meeting" , "Protest march" , "Display showing how something works" .)