What a climber may do for a bit of fun during a test (5)
I believe the answer is:
mount
'what a climber may do for a bit' is the definition.
The definition suggests a singular noun which matches the answer.
'fun during a test' is the wordplay.
'fun' becomes 'un' (I can't explain this - if you can you should give a lot more credence to this answer).
'during' is an insertion indicator.
'a test' becomes 'MOT' (as in an MOT test for a vehicle).
'un' put within 'mot' is 'MOUNT'.
'of' is the link.
Can you help me to learn more?
(Other definitions for mount that I've seen before include "Horse; climb" , "St. Helens, for example" , "hill" , "- - Sinai" , "Increase" .)