A potentially strange pet, old boy (5,4)
I believe the answer is:
blood type
'a potentially strange pet old boy' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'strange pet old boy' is the wordplay.
'strange' indicates anagramming the letters.
'pet'+'old'+'boy'='petoldboy'
'petoldboy' anagrammed gives 'BLOOD TYPE'.
Can you help me to learn more?
(Other definitions for blood type that I've seen before include "A or B, for example?" , "such as Auden or Betjeman for starters" , "This might be A" , "Font Used By Vampires?" .)