One-dimensional retro part of transport infrastructure crossing small space (6)
I believe the answer is:
linear
'one-dimensional' is the definition.
(similar in meaning)
'retro part of transport infrastructure crossing small space' is the wordplay.
'retro' says the letters should be written backwards (retro is Latin for 'backwards').
'part of transport infrastructure' becomes 'rail' (rail is a type of transport).
'crossing' is an insertion indicator.
'small space' becomes 'en' (in printing, a space as wide as a letter 'n').
'rail' enclosing 'en' is 'raenil'.
'raenil' written backwards gives 'LINEAR'.
(Other definitions for linear that I've seen before include "Measured lengthwise" , "One-dimensional" , "Involving one dimension only" , "following straight path" , "Of a line or length" .)