… in earth, moving about hundred tons — it only allows travel in one direction (7)
I believe the answer is:
ratchet
'it only allows travel in one direction' is the definition.
(a rachet only allows motion in one direction)
'in earth moving about hundred tons' is the wordplay.
'in' indicates putting letters inside.
'moving about' indicates anagramming the letters.
'hundred' becomes 'c' (Roman numeral).
'tons' becomes 't'.
'earth' with letters rearranged gives 'rathe'.
'rathe' placed around 'c' is 'ratche'.
'ratche'+'t'='RATCHET'
(Other definitions for ratchet that I've seen before include "Device stopping backward movement" , "Device preventing turning backwards" , "One-way-motion toothed wheel" , "Toothed rack" , "Device promoting One Direction" .)