Block is in place (6)
I believe the answer is:
resist
'block' is the definition.
(thesaurus)
'is in place' is the wordplay.
'in' indicates putting letters inside.
'place' becomes 'rest' (resting is a kind of placing**).
'is' going inside 'rest' is 'RESIST'.
(Other definitions for resist that I've seen before include "Withstand or defy" , "Withstand, oppose" , "Take stand against, oppose" , "'Withstand, fight against (6)'" , "Put up fight, oppose forcefully" .)