Six running on the rails in maiden (6)
I believe the answer is:
virgin
'maiden' is the definition.
(thesaurus)
'six running on the rails in' is the wordplay.
'six' becomes 'vi' ('vi' can be a synonym of 'six').
'on the rails' suggests removing the centre (leaving just the edges or rails of the word).
'running' with its middle removed is 'rg'.
'vi'+'rg'+'in'='VIRGIN'
(Other definitions for virgin that I've seen before include "New" , "Pure" , "Unsullied" , "Maiden" , "Not yet used" .)