Stuff initially plain as day (3)
I believe the answer is:
pad
'stuff' is the definition.
(I know that pad is a more specific form of stuff)
'initially plain as day' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'plain as day' is 'PAD'.
(Other definitions for pad that I've seen before include "Sole" , "Writing block" , "In slang, one's flat or bedsitter" , "Bachelor's place?" , "suitable for writers" .)