It's horn is crumpled, out of line (5)
I believe the answer is:
rhino
'line' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'it's horn is crumpled out' is the wordplay.
'it's' becomes 'i' (this could be a standard abbreviation which I don't know about).
'is crumpled' indicates an anagram (a crumbled or damaged version of the letters).
'out' means one lot of letters goes inside another.
'horn' with letters rearranged gives 'rhno'.
'i' going within 'rhno' is 'RHINO'.
'of' is the link.
Can you help me to learn more?
(Other definitions for rhino that I've seen before include "Horned beast (abbr.)" , "old-fashioned bread" , "money in the past" , "Horned pachyderm" , "endangered creature" .)