Big old critter, a good specimen, turned to frown about playing around (10)
I believe the answer is:
megalosaur
'big old critter' is the definition.
The definition suggests a singular noun which matches the answer.
'a good specimen turned to frown about playing around' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'good' could be 'g' (abbreviation) and 'g' is found within the answer.
'about' could be 'a' (abbreviation in Chambers) and 'a' is located in the answer.
'around' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for megalosaur that I've seen is " primitive beast".)