Later the beam will be split by axe, finally (10)
I believe the answer is:
thereafter
'later' is the definition.
(I've seen this before)
'the beam will be split by axe finally' is the wordplay.
'beam' becomes 'rafter' (rafter is a kind of beam).
'will be split by' is an insertion indicator (some letters split a word into two).
'finally' suggests the final letters.
The last letter of 'axe' is 'e'.
'rafter' enclosing 'e' is 'reafter'.
'the'+'reafter'='THEREAFTER'
(Other definitions for thereafter that I've seen before include "Following that time" , "Next" , "Subsequently" , "later" .)