Thanks the unknown one for providing transport (4)
I believe the answer is:
taxi
'transport' is the definition.
(I know that taxi is a type of transport)
'thanks the unknown one' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'the unknown' becomes 'x' (algebra).
'one' becomes 'i' (Roman numeral).
'ta'+'x'+'i'='TAXI'
'for providing' acts as a link.
(Other definitions for taxi that I've seen before include "Hackney carriage" , "people carrier?" , "Danny DeVito sitcom" , "type of vehicle" , "Cab for hire" .)