It may be pressed to gain an admission (8)
I believe the answer is:
doorbell
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'be pressed to gain an admission' is the wordplay.
I cannot quite see how this works, but
'be' is found in the answer.
This explanation may well be incorrect...
'may' acts as a link.
Can you help me to learn more?
(Other definitions for doorbell that I've seen before include "Ring it to announce presence of arrival in bordello" , "Ring it for admittance" , "Bordello with signal fitted to entrance" , "Visitors ring it to signal to house occupant" , "What one rings to be let in" .)