Rage at first, later beam (6)
I believe the answer is:
rafter
'beam' is the definition.
(rafter is a kind of beam)
'rage at first later' is the wordplay.
'at first' suggests taking the first letters.
'later' becomes 'after' (I've seen this before).
The first letter of 'rage' is 'r'.
'r'+'after'='RAFTER'
(Other definitions for rafter that I've seen before include "See retired Aussie tennis player" , "Strong wooden beam for roof" , "person on floating logs" , "Sloping beam that supports the roof" , "support" .)