That which diminishes may be less than amusing (5)
I believe the answer is:
minus
'amusing' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for minus that I've seen before include "Take-away (sign)" , "Lacking; negative" , "Reduced by the subtraction of - thus?" , "'Take away' sign" , "Less, without" .)