In principle, ring after eleven in the morning (5)
I believe the answer is:
axiom
'in principle' is the definition.
'axiom' can be an answer for 'principle' (an axiom is a belief or principle). I am unsure of the 'in' bit.
'ring after eleven in the morning' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'after' is a charade indicator (letters next to each other).
'eleven' becomes 'xi' (Roman numerals).
'in the' means one lot of letters goes inside another.
'morning' becomes 'am' (as in 12:00 AM).
'o' put after 'xi' is 'xio'.
'xio' inserted 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" .)