Post or beam with one end knocked off (5)
I believe the answer is:
after
'post' is the definition.
(the prefix 'post-' means 'after')
'beam with one end knocked off' is the wordplay.
'beam' becomes 'rafter' (rafter is a kind of beam).
'with one end knocked off' means to remove the first letter (one end of the word is removed).
'rafter' with its initial letter taken off is 'AFTER'.
'or' is the link.
(Other definitions for after that I've seen before include "since" , "Behind; seeking" , "Following - behind" , "Subsequent to" , "Searching for" .)