You may pay it when you get on, of course (4)
I believe the answer is:
fare
'course' is the definition.
(thesaurus)
'you may pay it when you get on' is the wordplay.
I cannot quite understand how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found in the answer.
The remaining letters 'far' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for fare that I've seen before include "Food - money for travelling - get on" , "what the commuter will pay" , "Price of passage" , "Taxi passenger; food" , "Taxi charge; food" .)