He let pony play with plastic wrap (9)
I believe the answer is:
polythene
'plastic wrap' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'he let pony play' is the wordplay.
'play' indicates an anagram.
'he'+'let'+'pony'='heletpony'
'heletpony' is an anagram of 'POLYTHENE'.
'with' is the link.
Can you help me to learn more?
(Other definitions for polythene that I've seen before include "A lightweight thermoplastic" , "Synthetic resin" , "Strange telephony provides a lightweight material, used in packaging" , "A light plastic material used for wrapping" , "Packaging material -- telephony (anag)" .)