Acorns provided by strong breed of dog? (7)
I believe the answer is:
mastiff
'breed of dog?' is the definition.
'acorns provided by strong' is the wordplay.
'acorns' becomes 'mast' (botanical term for forest nuts).
'provided' becomes 'if'.
'by' means one lot of letters go next to another.
'strong' becomes 'f'.
'mast'+'if'+'f'='MASTIFF'
(Other definitions for mastiff that I've seen before include "Big dog" , "one of his best friends?" , "Powerful dog breed" , "Powerful breed of watchdog" , "Strong breed of dog" .)