Investigate where to pay (8)
I believe the answer is:
checkout
'pay' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'investigate where' is the wordplay.
'investigate' becomes 'check' (I've seen this before).
'where' becomes 'out' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'check'+'out'='CHECKOUT'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for checkout that I've seen before include "Where one pays" , "Exit with till" , "one of the facilities at supermarket" .)