Aspect ultimately tiresome in truth (5)
I believe the answer is:
facet
'aspect' is the definition.
(I know that aspect can be written as facet)
'ultimately tiresome in truth' is the wordplay.
'ultimately' says to take the final letters.
'in' is an insertion indicator.
'truth' becomes 'fact' (true facts).
The final letter of 'tiresome' is 'e'.
'e' going within 'fact' is 'FACET'.
(Other definitions for facet that I've seen before include "Angle" , "Small flat surface of cut gemstone" , "Aspect or feature of something" , "Small surface (of diamond)" , "Characteristic" .)