Aquatic creature apt to play around yard in addition to going outside (8)
I believe the answer is:
platypus
'aquatic creature' is the definition.
(animal that lives by water)
'apt to play around yard in addition to going outside' is the wordplay.
'to play around' indicates anagramming the letters (I've seen 'playing around' mean this (I've seen this in other clues)).
'yard' becomes 'y' (abbreviation).
'in' is an insertion indicator.
'addition' becomes 'plus' (I've seen this before).
'to going outside' means one lot of letters goes inside another (the word goes on the outside of inserted letters).
'apt' anagrammed gives 'atp'.
'atp' placed around 'y' is 'atyp'.
'atyp' inserted inside 'plus' is 'PLATYPUS'.
(Other definitions for platypus that I've seen before include "Australian animal with ducklike beak" , "Aquatic creature" , "Exotic animal" , "Australian mammal with duckbill" , "Primitive mammal, duck-billed" .)