To keep a broken pen in it, is absurd (5)
I believe the answer is:
inept
'absurd' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?
'to keep a broken pen in' is the wordplay.
'to keep' becomes 'it' (I can't explain this - if you can you should give a lot more credence to this answer).
'a broken' indicates an anagram.
'in' means one lot of letters goes inside another.
'pen' with letters rearranged gives 'nep'.
'it' placed around 'nep' is 'INEPT'.
'it is' is the link.
Can you help me to learn more?
(Other definitions for inept that I've seen before include "Awkward, clumsy" , "Silly or senseless" , "Maladroit" , "Wanting" , "Lacking skill" .)