Shy holding acid, causing alarm (9)
I believe the answer is:
startling
'causing alarm' is the definition.
I can't tell whether this definition defines the answer.
'shy holding acid' is the wordplay.
'shy' becomes 'sling' (I've seen this in other clues).
'holding' means one lot of letters goes inside another.
'acid' becomes 'tart'.
'sling' placed around 'tart' is 'STARTLING'.
Can you help me to learn more?
(Other definitions for startling that I've seen before include "Extraordinary" , "Causing surprise" , "Very surprising, astonishing" , "Alarming due to rattlings" , "staggering" .)