Teachers' union is interested in it (4)
I believe the answer is:
into
'teachers' union is interested' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'in it' is the wordplay.
'it' becomes 'to' (I am not sure about this - if you are sure you should believe this answer much more).
'in'+'to'='INTO'
Can you help me to learn more?
(Other definitions for into that I've seen before include "A fan of" , "Expression of division" , "Curious about" , "Two -- six is three" , "Keen on (informal)" .)