Crush end of thumb in doorpost (4)
I believe the answer is:
jamb
'doorpost' is the definition.
(post holding up a door)
'crush end of thumb' is the wordplay.
'crush' becomes 'jam' (synonyms).
'end of' indicates one should take the final letters.
The final letter of 'thumb' is 'b'.
'jam'+'b'='JAMB'
'in' acts as a link.
(Other definitions for jamb that I've seen before include "part of doorway?" , "Doorpost" , "Part of door frame" , "bit on the side?" , "Vertical part to a doorframe" .)