How old is that frock? Ask the horse! (8)
I believe the answer is:
dressage
'horse' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'how old is that frock? ask' is the wordplay.
I cannot really see how this works, but
'frock?' could be 'dress' ('dress' can be a synonym of 'frock') and 'dress' is present in the answer.
'how old' could be 'age' (I've seen this before) and 'age' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'the' acts as a link.
Can you help me to learn more?
(Other definitions for dressage that I've seen before include "Riding skill" , "Art of training horses" , "The training of a horse in manoeuvres" , "Event for rider" , "equestrian activity" .)