More certain resistance stops one taking legal action (5)
I believe the answer is:
surer
'more certain' is the definition.
('sure' can be a synonym of 'certain')
'resistance stops one taking legal action' is the wordplay.
'resistance' becomes 'r' (abbreviation).
'stops' indicates putting letters inside.
'one taking legal action' becomes 'suer'.
'r' inserted inside 'suer' is 'SURER'.
(Other definitions for surer that I've seen before include "more right?" , "More certain and positive" , "faster" .)