Tick on the slate (6)
I believe the answer is:
credit
'tick' is the definition.
(I've seen this before)
'on the slate' is the wordplay.
I cannot really see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for credit that I've seen before include "Time to pay, or due recognition" , "Acknowledgement given for deed or quality" , "entry in cashbook" , "Believe; account entry" , "acknowledgement of liability" .)