You've not been going straight if you have to make this up (6)
I believe the answer is:
leeway
'you've not been going straight if you have to make this up' is the definition.
The definition suggests a singular noun which matches the answer.
'you have to make this up' is the wordplay.
'you' becomes 'ya' (non-standard spelling as in 'see ya later').
'have' says to put letters next to each other.
'to make' becomes 'weel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'this up' is a reversal indicator (in down clue: letters go upwards).
'ya'+'weel'='yaweel'
'yaweel' in reverse letter order is 'LEEWAY'.
Can you help me to learn more?
(Other definitions for leeway that I've seen before include "Margin, latitude, room" , "Drift to make up" , "Room to manoeuvre, latitude" , "Margin of safety" , "Amount of freedom" .)