Am exercising the rheumaticky dog (7)
I believe the answer is:
mastiff
'dog' is the definition.
(mastiff is a kind of dog)
'am exercising the rheumaticky' is the wordplay.
'exercising' is an anagram indicator.
'the rheumaticky' becomes 'stiff' (I am not sure about this - if you are sure you should believe this answer much more).
'am' with letters rearranged gives 'ma'.
'ma'+'stiff'='MASTIFF'
Can you help me to learn more?
(Other definitions for mastiff that I've seen before include "Powerful dog breed" , "Strong breed of dog" , "Powerful breed of watchdog" , "one of his best friends?" , "Big dog" .)