From then, insert the end into the internal roof beam (10)
I believe the answer is:
thereafter
'from then insert' is the definition.
I can't tell whether this definition defines the answer.
'the end into the internal roof beam' is the wordplay.
'end' says to take the final letters.
'into' means one lot of letters goes inside another.
'internal roof beam' becomes 'rafter' (I can't explain this - if you can you should believe this answer much more).
The final letter of 'the' is 'e'.
'the'+'rafter'='therafter'
'e' inserted within 'therafter' is 'THEREAFTER'.
Can you help me to learn more?
(Other definitions for thereafter that I've seen before include "Subsequently" , "later" , "Next" , "Following that time" .)