A singular line in transport (8)
I believe the answer is:
monorail
'transport' is the definition.
(I've seen this before)
'a singular line' is the wordplay.
'a singular' becomes 'mono' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'line' becomes 'rail' (synonyms).
'mono'+'rail'='MONORAIL'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for monorail that I've seen before include "Form of transport" , "train" , "Suspended railway" , "Hanging transport system" , "Type of railway" .)