Suffer among reactionary force of old (5)
I believe the answer is:
incur
'suffer' is the definition.
(to incur is to suffer or experience)
'among reactionary force of old' is the wordplay.
'among' becomes 'in' (I've seen this in other clues).
'reactionary' is a reversal indicator (reactions happen in the opposite direction).
'force of old' becomes 'RUC' (Royal Ulster Constabulary, old police force).
'ruc' backwards is 'cur'.
'in'+'cur'='INCUR'
(Other definitions for incur that I've seen before include "Assume" , "Provoke" , "Become subject to (eg, taxes)" , "Bring upon oneself - be liable for" , "Become liable to (eg, a penalty)" .)