Frank joins union in flexible arrangement (4,8)
I believe the answer is:
open marriage
'flexible arrangement' is the definition.
The definition suggests a singular noun which matches the answer.
'frank joins union' is the wordplay.
'frank' becomes 'open' (synonyms).
'joins' says to put letters next to each other.
'union' becomes 'marriage' (synonyms).
'open'+'marriage'='OPEN MARRIAGE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for open marriage that I've seen before include "Union in which the partners do not require fidelity" , "Union in which partners have great freedom" .)