Supplements found in coating of acid drops (4)
I believe the answer is:
adds
'supplements' is the definition.
(I've seen this in another clue)
'coating of acid drops' is the wordplay.
'coating of' means to remove the middle letters (only the letters that coat the word remain).
'drops' becomes 'ds' (I can't justify this - if you can you should give a lot more credence to this answer).
'acid' with its middle removed is 'ad'.
'ad'+'ds'='ADDS'
'found in' acts as a link.
Can you help me to learn more?
(Other definitions for adds that I've seen before include "Puts in; tacks on" , "Counts" , "Supplements" , "Appends; Totals" , "Does summer job" .)