The most common and most likely answer to this crossword clue is the 4 letter word MIND
I've seen this crossword clue in The Guardian
M
I
N
D
More clues leading to the result MIND
- Fear to interrupt child speaking? I know what you’re thinking
- Stage act showing fear when cornered by fellow in pit
- It’s shattering, what a reluctant wind player may do!
- Brain’s minimum diameter
- Member with independent opinion
- Watch out for intellect
- Doctor embraces home care
- Take umbrage at sexism in design houses
- Object to look after
- Takes care of survey — memory’s required
Buddy explains!

The straight part
We know that "opinion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case opinion = mind.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MIND mean?
MIND noun- The ability for rational thought.
- The ability to be aware of things.
- The ability to remember things.
- The ability to focus the thoughts.
- Somebody that embodies certain mental qualities.
- Judgment, opinion, or view.
- Desire, inclination, or intention.
- A healthy mental state.
- The non-material substance or set of processes in which consciousness, perception, affectivity, judgement, thinking, and will are based.
- Continual prayer on a dead person's behalf for a period after their death.
- (originally and chiefly in negative or interrogative constructions) To dislike, to object to; to be bothered by.
- To look after, to take care of, especially for a short period of time.
- (chiefly in the imperative) To make sure, to take care (that).
- To be careful about.
- Take note; used to point out an exception or caveat.
- To attend to, concern oneself with, heed, be mindful of.
- To remember.
- To have in mind; to intend.
- To put in mind; to remind.