Alarming possibility, however slight (8)
I believe the answer is:
marginal
'however slight' is the definition.
'marginal' can be an answer for 'slight' (thesaurus). I'm not sure about the 'however' bit.
'alarming possibility' is the wordplay.
'possibility' indicates an anagram (another possible arrangement of the letters).
'alarming' with letters rearranged gives 'MARGINAL'.
Can you help me to learn more?
(Other definitions for marginal that I've seen before include "Peripheral in alarming way" , "Of minor importance" , "On the fringes" , "Very close (decision)" , "On the edges of" .)