It is generally considered unacceptable for a setter to arbitrarily remove a capital letter which is required by the cryptic reading of a clue. So ‘Take one around nice course’ for RUN is no good, since ‘nice’ has to be ‘Nice’ for the wordplay (R + UN) to work. So how about ‘Pass river, one in Nice’, also for RUN? Superficially, that looks ok. But a check in Chambers tells us that R is an abbreviation for ‘River’ (with a capital letter), but not ‘river’.
We could ask ourselves “Is the abbreviation ever used in real life to indicate the complete word without the initial capital?” In this instance (for ‘R=river’) the answer seems to me a definite “No” – the abbreviation is only seen in the names of specific rivers (typically on maps) as eg ‘R. Thames’, and is never used generically. So saying ‘river’ when you mean ‘River’ is really no different to saying ‘nice’ when you mean ‘Nice’, and presumably shouldn’t be allowed.
Apart from ‘river’, such a prohibition would rule out several old favourites lacking an initial capital: ‘street’, ‘road’ and ‘lake’ from the ‘geographical’ group, as well as ‘society’ (for ‘S’) and ‘king’ (for ‘R’, though not for ‘K’, which is an abbreviation for ‘king’ in the context of chess or cards).
Knowing this, would I use, say, ‘river’ for R in a clue? Well, since I suspect that there isn’t a single solver out there who would bat an eyelid, I certainly wouldn’t reject a good clue purely to avoid it. I think it’s fair to say that some wordplay elements are so well established that it makes no sense to unilaterally boycott them, but this does show that even at a general level there is on occasion a lack of consistency when it comes to what is deemed allowable and what is not.