Stillness in one train crashing round east (7)
I believe the answer is:
inertia
'stillness' is the definition.
(I've seen this before)
'in one train crashing round east' is the wordplay.
'in' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'crashing' indicates an anagram.
'round' indicates putting letters inside.
'east' becomes 'e' (synonyms).
'train' anagrammed gives 'nrtia'.
'i' placed within 'nrtia' is 'inrtia'.
'inrtia' going around 'e' is 'INERTIA'.
(Other definitions for inertia that I've seen before include "Passiveness" , "Sort of reel" , "inclination not to budge" , "Stillness" , "Resistance to movement" .)