Unlikely hint re: nausea in debility (12)
I believe the answer is:
neurasthenia
'debility' is the definition.
(I have seen 'Nervous debility ' mean 'neurasthenia' so perhaps 'debility' could also mean 'neurasthenia')
'unlikely hint re nausea' is the wordplay.
'unlikely' indicates an anagram (an unlikely spelling of the letters).
'hint'+'re'+'nausea'='hintrenausea'
'hintrenausea' anagrammed gives 'NEURASTHENIA'.
'in' is the link.
(Other definitions for neurasthenia that I've seen before include "Nervous condition" , "stress condition" , "psychological disorder?" , "Nervous debility" .)