Some unpopular opinions about certain type of vaccine (4)
I believe the answer is:
oral
'type of vaccine' is the definition.
(I've seen this in another clue)
'some unpopular opinions about' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'about' is a reversal indicator.
'laro' is hidden within 'unpopular opinions'.
'laro' written backwards gives 'ORAL'.
'certain' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for oral that I've seen before include "Spoken rather than written" , "Spoken test" , "Concerning the mouth" , "Type of exam" , "Relating to speech" .)