Something elementary in Reagan campaign slogan? (8)
I believe the answer is:
electron
'campaign slogan?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'something elementary in reagan' is the wordplay.
I cannot quite see how this works, but
'reagan' could be 'ron' (I've seen this in another clue) and 'ron' is found within the answer.
The remaining letters 'elect' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for electron that I've seen before include "Negatively charged particle" , "Particle found in all atoms" , "Subatomic particle -- enrol etc (anag)" , "There's not much to it" , "Negatively-charged particle" .)