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 justify this - if you can you should believe this answer much more).
'during' is an insertion indicator.
'a test' becomes 'mot'.
'un' inserted into '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" .)