He has lines to deliver, of course (7)
I believe the answer is:
postman
'he has lines to deliver of course' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he has lines to deliver of' is the wordplay.
I cannot quite see how this works, but
'he' could be 'man' (I've seen this in another clue) and 'man' is found in the answer.
'of' could be 'o' and 'o' is located in the answer.
The remaining letters 'pst' 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 postman that I've seen before include "No stamp needed, stangely, for this job" , "one delivers" , "Letter deliverer" , "Chap with letters" , "Male mail carrier" .)