Underground shelter arrived at with input from Republican (6)
I believe the answer is:
grotto
'underground shelter' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'arrived at with input from republican' is the wordplay.
'arrived at' becomes 'got to' ('get to' can be a synonym of 'arrive at').
'with' indicates putting letters inside.
'input from republican' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'gotto' going around 'r' is 'GROTTO'.
Can you help me to learn more?
(Other definitions for grotto that I've seen before include "Santa's cave" , "A picturesque cave as in Lourdes (6)" , "Small cave as at Lourdes" , "Small picturesque cave" , "Pretty cave" .)