Very keen on some twin towns (4)
I believe the answer is:
into
'very keen on' is the definition.
'some twin towns' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'INTO' is hidden within 'twin towns'.
(Other definitions for into that I've seen before include "Liking" , "digging" , "Enthusiastic about" , "A fan of" , "To a position inside" .)