Keen to take mum to the capital of Denmark (3)
I believe the answer is:
mad
'keen' is the definition.
(both can mean enthusiastic about)
'mum to the capital of denmark' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'to the capital of' says to take the initial letters.
The initial letter of 'denmark' is 'd'.
'ma'+'d'='MAD'
'to take' acts as a link.
(Other definitions for mad that I've seen before include "Idiotic" , "lunatic" , "Berserk" , "Deranged" , "Angry or insane" .)