Railmen moving ore
I believe the answer is:
mineral
'ore' is the definition.
(I've seen this before)
'railmen moving' is the wordplay.
'moving' indicates an anagram.
'railmen' is an anagram of 'MINERAL'.
(Other definitions for mineral that I've seen before include "E.g. bauxite, quartz" , "Substance needed for health; ore" , "Valuable ore" , "Made from inorganic matter" , "Gypsum or magnesium, say" .)