Is he grumbling about the old-fashioned transport? (7)
I believe the answer is:
Here is my best explanation:
'is he grumbling about the old-fashioned transport?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is the entire clue.
(Other definitions for growler that I've seen before include "Iceberg", "Perhaps bear", "Dog perhaps" and "Unfriendly dog".)