Helps a lieutenant to guard key for compound (4,4)
I believe the answer is:
acid salt
'compound' is the definition.
(chemical compound)
'helps a lieutenant to guard key' is the wordplay.
'helps' becomes 'aids' ('aid' can be a synonym of 'help').
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'to guard' means one lot of letters goes inside another.
'key' becomes 'c' (musical key).
'aids'+'a'+'lt'='aidsalt'
'aidsalt' enclosing 'c' is 'ACID SALT'.
'for' acts as a link.
(Other definitions for acid salt that I've seen before include "Compound such as potassium hydrogen sulphate" , "Compound; aids talc (anag.)" .)