Cut top off beam later on (5)
I believe the answer is:
after
'later on' is the definition.
(I've seen this before)
'cut top off beam' is the wordplay.
'cut top off' means to remove the first letter.
'beam' becomes 'rafter' (rafter is a kind of beam).
'rafter' with its first letter taken away is 'AFTER'.
(Other definitions for after that I've seen before include "Looking for" , "Coming later in time" , "back along > deck" , "In the time following" , "Searching for" .)