Block chimney, applying pressure (4)
I believe the answer is:
lump
'block' is the definition.
(thesaurus)
'chimney applying pressure' is the wordplay.
'chimney' becomes 'lum' (I've seen this in other clues).
'applying' says to put letters next to each other.
'pressure' becomes 'p' (abbreviation).
'lum'+'p'='LUMP'
(Other definitions for lump that I've seen before include "Shapeless mass - swelling" , "dullard" , "growth" , "Protuberance or swelling" , "Small solid mass" .)