Text written about a pet dog (7)
I believe the answer is:
mastiff
'dog' is the definition.
(breed of dog)
'text written about a pet' is the wordplay.
'text' becomes 'MS' (short for manuscript).
'written about' indicates putting letters inside.
'pet' becomes 'tiff' (I've seen this before).
'ms' enclosing 'a' is 'mas'.
'mas'+'tiff'='MASTIFF'
(Other definitions for mastiff that I've seen before include "Powerful, smooth-coated dog used for security work" , "one of his best friends?" , "Large short-haired dog breed" , "Big dog" , "Powerful dog breed" .)