Help to bottle cold substance that's corrosive (4)
I believe the answer is:
acid
'corrosive' is the definition.
(I've seen this in another clue)
'help to bottle cold substance' is the wordplay.
'help' becomes 'aid' (synonyms).
'to bottle' means one lot of letters goes inside another (bottle can mean to contain).
'cold substance' becomes 'c' (C is an example. I am not sure about the 'cold' bit.).
'aid' going around 'c' is 'ACID'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for acid that I've seen before include "Sour - LSD" , "See 4 Across" , "Sarcastic (could be sulphuric?)" , "Chemical compound" , "Opposite of alkali; sour" .)