Big hole in smalls? A boy's coat is needed for starters! (5)
I believe the answer is:
abyss
'big hole' is the definition.
(an abyss is a large chasm or pit)
'smalls? a boy's coat is needed for starters' is the wordplay.
I cannot really see how this works, but
'smalls?' could be 'ss' (two S's - as in Small clothing) and 'ss' is located in the answer.
'a' is within the answer.
'boy' could be 'b' (abbreviation used in schools) and 'b' is present in the answer.
A single letter 'y' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for abyss that I've seen before include "Very large and deep chasm" , "Deep gulf" , "ocean depth" , "depths of sea" , "Measureless chasm" .)