It makes an admission (5)
I believe the answer is:
inlet
'admission' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'it makes an' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for inlet that I've seen before include "Bay, cove" , "access point" , "An entrance or opening" , "Fiord" , "Means of entry" .)