Stubbs for one was about to enter without warning (8)
I believe the answer is:
unawares
'without warning' is the definition.
(thesaurus)
'stubbs for one was about to enter' is the wordplay.
'stubbs for' becomes 'una' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' becomes 're' (regarding).
'to enter' means one lot of letters goes inside another.
'was' placed around 're' is 'wares'.
'una'+'wares'='UNAWARES'
Can you help me to learn more?
(Other definitions for unawares that I've seen before include "without realising" , "in the dark" , "Warn Esau (anag)" , "Be taken thus if off-guard" , "Unexpectedly, without noticing in new auras" .)