A tree drops on author (5)
I believe the answer is:
adams
'author' is the definition.
(author Douglas Adams or Richard Adams)
'a tree drops' is the wordplay.
'tree drops' becomes 'dams' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'dams'='ADAMS'
'on' acts as a link.
(Other definitions for adams that I've seen before include "Ansel --, US landscape photographer" , "Gaolers" , "revolutionary leader" , "Of untidy appearance" , "Second and sixth US president" .)