Greengage, look at post 39, and also consider how the remark on the position of the 8 letter word (of which the 4 letter word will make part) might be interpreted. The 4 letter word is in plain view and once you see the solution you’ll know it’s right.
16 - term for a particular phase of an opera, not an act, unclued letters involved
18 - Definition is benefit, unclued letter in clue
19 - Definition is constituent of an oil, unclued letter in clue, think latin for well (done) to start with