Very obvious line penned by gag writer with only one good set (4,5)
I believe the answer is:
writ large
'very obvious' is the definition.
(I've seen this before)
'line penned by gag writer with only one good set' is the wordplay.
I cannot quite see how this works, but
'line' could be 'l' (used when specifying particular lines from a poem) and 'l' is found in the answer.
'writer' could be 'i' (the person who is writing these clues) and 'i' is found within the answer.
'with' could be 'w' (short for with) and 'w' is found in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is present in the answer.
'good' could be 'g' (abbreviation) and 'g' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for writ large that I've seen before include "very obvious" , "Emphasised" , "on a mass scale" , "exaggerated" .)