Sign my post, for a change, 'Mike' (7)
I believe the answer is:
symptom
'sign' is the definition.
(symptom is a kind of sign)
'my post for a change mike' is the wordplay.
'for a change' indicates anagramming the letters.
'mike' becomes 'm' (phonetic alphabet: alpha, bravo, charlie etc.).
'my'+'post'='mypost'
'mypost' anagrammed gives 'sympto'.
'sympto'+'m'='SYMPTOM'
(Other definitions for symptom that I've seen before include "Feature of a disease" , "Sign; indication" , "Troublesome sign" , "It alerts one to illness" , "Indication of illness" .)