Animal planned to explode in fairground attraction (8,4)
I believe the answer is:
shetland pony
'animal' is the definition.
(type of animal)
'planned to explode in fairground attraction' is the wordplay.
'explode' indicates anagramming the letters.
'in' indicates putting letters inside.
'fairground attraction' becomes 'shy' (as in a coconut shy).
'planned'+'to'='plannedto'
'plannedto' is an anagram of 'etlandpon'.
'etlandpon' placed within 'shy' is 'SHETLAND PONY'.
(Other definitions for shetland pony that I've seen before include "Small, shaggy-maned animal" , "Steed" , "Small breed of horse" , "Rough-coated mount" .)