More dangerous rioter heartlessly surrounds runner (7)
I believe the answer is:
riskier
'more dangerous' is the definition.
('risky' can be similar in meaning to 'dangerous')
'rioter heartlessly surrounds runner' is the wordplay.
'heartlessly' suggests removing the centre.
'surrounds' indicates putting letters inside.
'runner' becomes 'ski' (something which 'runs', ie moves quickly on snow).
'rioter' with its centre taken out is 'rier'.
'rier' enclosing 'ski' is 'RISKIER'.
(Other definitions for riskier that I've seen before include "less safe" , "More likely to cause loss" , "More chancy" .)