Extremely shy politician gets a cat, a sign of disease (7)
I believe the answer is:
symptom
'a sign of disease' is the definition.
(I've seen this before)
'extremely shy politician gets a cat' is the wordplay.
'extremely' means to remove the middle letters (letters on the extremes of the word are used).
'politician' becomes 'MP' (Member of Parliament).
'gets' means one lot of letters go next to another.
'a cat' becomes 'tom' (a tom is a male cat).
'shy' with its middle taken out is 'sy'.
'sy'+'mp'+'tom'='SYMPTOM'
(Other definitions for symptom that I've seen before include "Prodrome" , "Sign of disease" , "sign all's not well" , "Indicator of disease" , "Feature of a disease" .)