Place to learn about marriage (5)
I believe the answer is:
union
'marriage' is the definition.
(I know that marriage can be written as union)
'place to learn about' is the wordplay.
'place to learn' becomes 'uni'.
'about' becomes 'on' (as in a book 'on' a certain topic).
'uni'+'on'='UNION'
(Other definitions for union that I've seen before include "could be seen in middle of Grand Canal" , "Organisation of employees for bargaining purposes" , "Fusion" , "Coupling, merger" , "political alliance" .)