A bit of catastrophe increases keenness! (5)
I believe the answer is:
strop
'keenness' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'a bit of catastrophe increases' is the wordplay.
'a bit of' says the answer is hidden in the clue (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'STROP' is hidden in the letters of 'catastrophe increases'.
Can you help me to learn more?
(Other definitions for strop that I've seen before include "Bad temper - sharpen (a razor)" , "Sharpen on piece of leather" , "rage" , "Razor sharpener" , "temper tantrum" .)