Innocent inventor of decimal system this writer had talked of (4-4)
I believe the answer is:
dewy-eyed
'innocent' is the definition.
(I've seen this before)
'inventor of decimal system this writer had talked of' is the wordplay.
'inventor of decimal system' becomes 'dewy' (I can't explain this - if you can you should give a lot more credence to this answer).
'this writer had' becomes 'id' ('I'd' refers to the person writing the clues).
'talked of' indicates a 'sounds like' (homophone) clue.
'id' sounds like 'eyed'.
'dewy'+'eyed'='DEWY-EYED'
(Other definitions for dewy-eyed that I've seen before include "Exhibiting childlike credulity" , "Lovingly, naively trustful" , "Seeming rather tearful" , "Sentimental or innocently trusting, being ocularly moist?" , "Romantically naive - on the verge of tears" .)