Evil infiltrating usual tower of strength (6)
I believe the answer is:
pillar
'tower of strength' is the definition.
(thesaurus)
'evil infiltrating usual' is the wordplay.
'evil' becomes 'ill' (I've seen this in another clue).
'infiltrating' is an insertion indicator.
'usual' becomes 'par' (par is an average golf score).
'ill' inserted into 'par' is 'PILLAR'.
(Other definitions for pillar that I've seen before include "Vertical stone support" , "Tall vertical structure, of stone say" , "Column, support" , "Supporter" , "Upright post" .)