Improve railway leading to factory (8)
I believe the answer is:
refinery
'factory' is the definition.
(thesaurus)
'improve railway leading' is the wordplay.
'improve' becomes 'refine' (refining is a kind of improving**).
'railway' becomes 'ry' (abbreviation for railway).
'leading' is a charade indicator (letters next to each other).
'refine'+'ry'='REFINERY'
'to' is the link.
(Other definitions for refinery that I've seen before include "Where oil is processed" , "Purification plant" , "Industrial plant for purifying crude substances" , "Plant for purifying, oil say" , "Industrial plant for purifying a crude substance" .)