Reversing chromosome constituent can be a plus (3)
I believe the answer is:
and
'plus' is the definition.
(I've seen this before)
'reversing chromosome constituent can be a' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found in the answer.
A single letter 'd' remains which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for and that I've seen before include "Most-used conjunction in English" , "A very common conjunction" , "Word to connect" , "Plus" , "See 20" .)