Soldier about to enter Reading after riot (9)
I believe the answer is:
grenadier
'soldier' is the definition.
(historical term for a grenade-carrying soldier)
'about to enter reading after riot' is the wordplay.
'about to' becomes 're' (I've seen this in other clues).
'enter' means one lot of letters goes inside another.
'after riot' indicates anagramming the letters (letters riot from their original order).
'reading' is an anagram of 'gnadier'.
're' put into 'gnadier' is 'GRENADIER'.
(Other definitions for grenadier that I've seen before include "Soldier - in red gear (anag)" , "African flier" , "bomb-thrower" , "A household infantryman" , "Re-reading for the deep-sea fish" .)