It's taken for granted at ten to ten in the morning (5)
I believe the answer is:
axiom
'it's taken for granted' is the definition.
(I've seen this before)
'ten to ten in the morning' is the wordplay.
'ten' becomes 'X' (Roman numeral).
'to ten' becomes 'IO' (IO resembles 10).
'in the' is an insertion indicator.
'morning' becomes 'am'.
'x'+'io'='xio'
'xio' put inside 'am' is 'AXIOM'.
'at' is the link.
(Other definitions for axiom that I've seen before include "Generally accepted proposition" , "Generally accepted principle" , "A saying that's widely accepted as true" , "Clearly true statement" , "Statement regarded as self-evidently true" .)