One of the signs that motorists have to go on (2,7)
I believe the answer is:
no parking
'one of the signs' is the definition.
The definition suggests an adjective which matches the answer.
'motorists have to go on' is the wordplay.
'motorists have' becomes 'parking' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to' means one lot of letters go next to another (I've seen this in other clues).
'go' indicates an anagram (I've seen 'went' mean this ('go' can mean move)).
'on' with letters rearranged gives 'no'.
'parking' put after 'no' is 'NO PARKING'.
'that' is the link.
Can you help me to learn more?
(Another definition for no parking that I've seen is " not a welcoming sign".)