After time, 'Swallows and Amazons' author almost makes one beam (7)
I believe the answer is:
transom
'one beam' is the definition.
(thesaurus)
'after time swallows and amazons author almost' is the wordplay.
'after' says to put letters next to each other.
'time' becomes 't'.
'swallows and amazons author' becomes 'ransome' (I've seen this before).
'almost' means to remove the last letter (most of the word but not all of it).
'ransome' with its last letter taken off is 'ransom'.
't'+'ransom' is 'TRANSOM'.
'makes' is the link.
(Other definitions for transom that I've seen before include "Horizontal bar across window" , "Crossbar over a window" , "timber for doorframe" , "Cross-member of boat or window" , "Bar above window or door" .)