This will cause ill-health - Dianas needing relaxation (7)
I believe the answer is:
disease
'this' is the definition.
The definition suggests a singular noun which matches the answer.
'ill-health dianas needing relaxation' is the wordplay.
'ill health dianas' becomes 'dis' (I can't explain this - if you can you should believe this answer much more).
'needing' means one lot of letters go next to another.
'relaxation' becomes 'ease' (synonyms).
'dis'+'ease'='DISEASE'
'will cause' is the link.
Can you help me to learn more?
(Other definitions for disease that I've seen before include "affection" , "Contagion" , "scrapie for example" , "scurvy?" , "Sickness" .)