Creed not one to criticise harshly (4)
I believe the answer is:
slam
'criticise harshly' is the definition.
'creed not one' is the wordplay.
'creed' becomes 'islam' (I've seen this before).
'not' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'islam' with 'i' taken out is 'SLAM'.
'to' is the link.
(Other definitions for slam that I've seen before include "Shut loudly and forcibly" , "ambitious bid" , "Shut hard" , "Criticise - bang - very good result at bridge" , "Shut violently" .)