Printer in police station? One of two up for approval (5)
I believe the answer is:
thumb
'printer in police station?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'one of two up for approval' is the wordplay.
'one of two up for' becomes 'thu' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'approval' becomes 'MB' (MB is an example).
'thu'+'mb'='THUMB'
Can you help me to learn more?
(Other definitions for thumb that I've seen before include "Hitch-hiker's tool" , "Big digit" , "Hand digit" , "Hitch (a lift)" , "Travel by . . . . . - hitchhike" .)