One or two up for approval (5)
I believe the answer is:
thumb
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'two up for approval' is the wordplay.
I cannot quite understand how this works, but
'approval' could be 'mb' (MB is an example) and 'mb' is located in the answer.
This explanation may well be incorrect...
'or' acts as a link.
Can you help me to learn more?
(Other definitions for thumb that I've seen before include "Big digit" , "Tom ___ , traditional hero of English folklore" , "Tom, for example" , "Second part of 12" , "Quickly turn pages" .)