One may show press instruction at an opening (8)
I believe the answer is:
doorbell
'one may' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'press instruction at an opening' is the wordplay.
'press instruction' becomes 'door' (I am not sure about this - if you are sure you should believe this answer much more).
'at' says to put letters next to each other.
'an opening' becomes 'bell' (bell is a kind of opening).
'door'+'bell'='DOORBELL'
'show' is the link.
(Other definitions for doorbell that I've seen before include "Ring it to announce presence of arrival in bordello" , "What one rings to be let in" , "Visitors ring it to signal to house occupant" , "Push button to ring" , "Bordello with signal fitted to entrance" .)