Only 50% chance — disastrous for Kingfisher (7)
I believe the answer is:
halcyon
'kingfisher' is the definition.
(I've seen this before)
'only 50 chance disastrous' is the wordplay.
'50' means to take half the letters.
'disastrous' is an anagram indicator.
'chance' cut in half is 'cha'.
'only'+'cha'='onlycha'
'onlycha' with letters rearranged gives 'HALCYON'.
'for' acts as a link.
(Other definitions for halcyon that I've seen before include "Any loch? (anag)" , "Happy and carefree" , "Calm and peaceful" , "Prosperous" , "Kingfisher genus" .)