Am exercising the rheumaticky dog (7)
I believe the answer is:
mastiff
'dog' is the definition.
(breed of dog)
'am exercising the rheumaticky' is the wordplay.
'exercising' indicates an anagram (letters exercise or move around).
'the rheumaticky' becomes 'stiff' (I am not sure about this - if you are sure you should believe this answer much more).
'am' anagrammed 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" .)