Lion is a problem in assignation (7)
I believe the answer is:
liaison
'assignation' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'lion is a problem' is the wordplay.
'problem' indicates an anagram.
'lion'+'is'+'a'='lionisa'
'lionisa' is an anagram of 'LIAISON'.
'in' is the link.
Can you help me to learn more?
(Other definitions for liaison that I've seen before include "Contact between groups" , "Communication between people or organisations" , "carryon" , "An affair or mediation" , "Mil. coordination; illicit affair" .)