It normally completes the written enquiry (8,4)
I believe the answer is:
question mark
'it normally completes the written' is the definition.
The definition suggests a singular noun which matches the answer.
'enquiry' is the wordplay.
I cannot really understand how this works, but
'enquiry' could be 'question' (synonyms) and 'question' is found in the answer.
The remaining letters 'mark' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for question mark that I've seen before include "Not believe the gospel" , "What ends this" , "Query" , "Expression of uncertainty" .)