Half of capital being lost is a nuisance (4)
I believe the answer is:
pest
'a nuisance' is the definition.
(I've seen this before)
'half of capital' is the wordplay.
'half of' suggests halving the letters.
'capital' becomes 'budapest' (Budapest is an example).
'budapest' cut in half is 'PEST'.
'being lost is' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Other definitions for pest that I've seen before include "Troublesome man or animal" , "Constant nuisance" , "Annoying creature" , "A nuisance - step on it" , "Annoyance" .)