Priest offers shelter to accident's first responder (7)
I believe the answer is:
reactor
'priest' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'shelter to accident's first responder' is the wordplay.
I cannot quite see how this works, but
'to' is found in the answer.
'first' could be 'a' (first letter of the alphabet) and 'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'offers' acts as a link.
Can you help me to learn more?
(Other definitions for reactor that I've seen before include "Nuclear power source" , "See 3 Down" , "Nuclear apparatus for producing energy" , "power supplier" , "Device to produce energy by nuclear fission" .)