Clean up what used to be the capital of Hungary (4)
I believe the answer is:
wash
'clean up' is the definition.
(I've seen this in another clue)
'used to be the capital of hungary' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'the capital of' says to take the initial letters.
The first letter of 'hungary' is 'h'.
'was'+'h'='WASH'
'what' is the link.
(Other definitions for wash that I've seen before include "Rough water left by a moving boat" , "shower" , "Of animals, cleanse by licking" , "Bear investigation" , "Stand up" .)