Buried in baby's sand pit (5)
I believe the answer is:
abyss
'pit' is the definition.
(I've seen this in another clue)
'buried in baby's sand' is the wordplay.
'buried in' indicates the answer is hidden within the clue.
'ABYSS' is hidden within 'babys sand'.
(Other definitions for abyss that I've seen before include "Very large and deep chasm" , "Hell; hole" , "Deep hole, seems bottomless" , "Bottomless pit" , "something rather deep" .)