Notebooks unopened? They're probably out swimming and running around (6)
I believe the answer is:
otters
'they're probably out swimming and running around' is the definition.
(otters are aquatic animals)
'notebooks unopened?' is the wordplay.
'notebooks' becomes 'jotters' (jotter is a kind of notebook).
'unopened?' means to remove the first letter (I've seen 'unopened' mean this).
'jotters' with its first letter taken away is 'OTTERS'.
(Other definitions for otters that I've seen before include "Dark brown freshwater mammals" , "Mammals with webbed feet" , "web-footed creatures" , "River-dwelling mammals" , "Water animals with webbed feet and flat tails" .)