Excessive inflation will destroy it (7)
I believe the answer is:
balloon
'excessive inflation' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'will destroy it' is the wordplay.
I cannot really understand how this works, but
'will' could be 'll' (contracted form, as in 'he'll' - 'he will') and 'll' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for balloon that I've seen before include "In which Charlie Brown expressed himself" , "Inflatable sphere, may contain hot air" , "Inflatable thin rubber ball" , "See 21" , "Bag filled with gas or air" .)