Link with half-life as ion disintegrates (7)
I believe the answer is:
liaison
'link' is the definition.
(I know that link can be written as liaison)
'half-life as ion disintegrates' is the wordplay.
'half-life' becomes 'li' (half of word 'life').
'disintegrates' indicates an anagram (I've seen 'disintegrated' mean this).
'as'+'ion'='asion'
'asion' with letters rearranged gives 'aison'.
'li'+'aison'='LIAISON'
'with' acts as a link.
(Other definitions for liaison that I've seen before include "Communication between organisations - or an illicit affair" , "Intimate relationship" , "Cooperation between people or organisations" , "Communication between people or organisations" , "Communication between groups or units" .)