In principle ring after eleven in the morning (5)
I believe the answer is:
'in principle' is the definition.
'axiom' can be an answer for 'principle' (an axiom is a belief or principle). I'm unsure of the 'in' bit.
'ring after eleven in the morning' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'after' means one lot of letters go next to another.
'eleven' becomes 'xi' (Roman numerals).
'in the' is an insertion indicator.
'morning' becomes 'am' (as in 12:00 AM).
'o' put after 'xi' is 'xio'.
'xio' going within 'am' is 'AXIOM'.
Can you help me to learn more?
(Other definitions for axiom that I've seen before include "Accepted statement taken as self-evidently true" , "Statement regarded as self-evidently true" , "saw" , "Accepted general truth or principle" , "truthful statement" .)